4.3 Dependencies

Your typical enterprise application is not made up of a single object (or bean in the Spring parlance). Even the simplest of applications will no doubt have at least a handful of objects that work together to present what the end-user sees as a coherent application. This next section explains how you go from defining a number of bean definitions that stand-alone, each to themselves, to a fully realized application where objects work (or collaborate) together to achieve some goal (usually an application that does what the end-user wants).

4.3.1 Injecting dependencies

The basic principle behind Dependency Injection (DI) is that objects define their dependencies (that is to say the other objects they work with) only through constructor arguments, arguments to a factory method, or properties which are set on the object instance after it has been constructed or returned from a factory method. Then, it is the job of the container to actually inject those dependencies when it creates the bean. This is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself being in control of instantiating or locating its dependencies on its own using direct construction of classes, or something like the Service Locator pattern.

It becomes evident upon usage that code gets much cleaner when the DI principle is applied, and reaching a higher grade of decoupling is much easier when objects do not look up their dependencies, but are provided with them (and additionally do not even know where the dependencies are located and of what concrete class they are). DI exists in two major variants, namely Constructor Injection and Setter Injection.

4.3.1.1 Constructor Injection

Constructor-based DI is effected by invoking a constructor with a number of arguments, each representing a dependency. Additionally, calling a static factory method with specific arguments to construct the bean, can be considered almost equivalent, and the rest of this text will consider arguments to a constructor and arguments to a static factory method similarly. Find below an example of a class that could only be dependency injected using constructor injection. Notice that there is nothing special about this class.

public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on a MovieFinder
    private MovieFinder movieFinder;

    // a constructor so that the Spring container can 'inject' a MovieFinder
    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
    
    // business logic that actually 'uses' the injected MovieFinder is omitted...
}
Constructor Argument Resolution

Constructor argument resolution matching occurs using the argument's type. If there is no potential for ambiguity in the constructor arguments of a bean definition, then the order in which the constructor arguments are defined in a bean definition is the order in which those arguments will be supplied to the appropriate constructor when it is being instantiated. Consider the following class:

package x.y;

public class Foo {

    public Foo(Bar bar, Baz baz) {
        // ...
    }
}

There is no potential for ambiguity here (assuming of course that Bar and Baz classes are not related in an inheritance hierarchy). Thus the following configuration will work just fine, and you do not need to specify the constructor argument indexes and / or types explicitly.

<beans>
    <bean name="foo" class="x.y.Foo">
        <constructor-arg>
            <bean class="x.y.Bar"/>
        </constructor-arg>
        <constructor-arg>
            <bean class="x.y.Baz"/>
        </constructor-arg>
    </bean>
</beans>

When another bean is referenced, the type is known, and matching can occur (as was the case with the preceding example). When a simple type is used, such as <value>true<value>, Spring cannot determine the type of the value, and so cannot match by type without help. Consider the following class:

package examples;

public class ExampleBean {

    // No. of years to the calculate the Ultimate Answer
    private int years;

    // The Answer to Life, the Universe, and Everything
    private String ultimateAnswer;

    public ExampleBean(int years, String ultimateAnswer) {
        this.years = years;
        this.ultimateAnswer = ultimateAnswer;
    }
}
Constructor Argument Type Matching

The above scenario can use type matching with simple types by explicitly specifying the type of the constructor argument using the 'type' attribute. For example:

<bean id="exampleBean" class="examples.ExampleBean">
  <constructor-arg type="int" value="7500000"/>
  <constructor-arg type="java.lang.String" value="42"/>
</bean>
Constructor Argument Index

Constructor arguments can have their index specified explicitly by use of the index attribute. For example:

<bean id="exampleBean" class="examples.ExampleBean">
  <constructor-arg index="0" value="7500000"/>
  <constructor-arg index="1" value="42"/>
</bean>

As well as solving the ambiguity problem of multiple simple values, specifying an index also solves the problem of ambiguity where a constructor may have two arguments of the same type. Note that the index is 0 based.

4.3.1.2 Setter Injection

Setter-based DI is realized by calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

Find below an example of a class that can only be dependency injected using pure setter injection. Note that there is nothing special about this class... it is plain old Java.

public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on the MovieFinder
    private MovieFinder movieFinder;

    // a setter method so that the Spring container can 'inject' a MovieFinder
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // business logic that actually 'uses' the injected MovieFinder is omitted...
}

The BeanFactory supports both of these variants for injecting dependencies into beans it manages. (It in fact also supports injecting setter-based dependencies after some dependencies have already been supplied via the constructor approach.) The configuration for the dependencies comes in the form of a BeanDefinition, which is used together with PropertyEditor instances to know how to convert properties from one format to another. However, most users of Spring will not be dealing with these classes directly (that is programmatically), but rather with an XML definition file which will be converted internally into instances of these classes, and used to load an entire Spring IoC container instance.

Bean dependency resolution generally happens as follows:

  1. The BeanFactory is created and initialized with a configuration which describes all the beans. (Most Spring users use a BeanFactory or ApplicationContext implementation that supports XML format configuration files.)

  2. Each bean has dependencies expressed in the form of properties, constructor arguments, or arguments to the static-factory method when that is used instead of a normal constructor. These dependencies will be provided to the bean, when the bean is actually created.

  3. Each property or constructor argument is either an actual definition of the value to set, or a reference to another bean in the container.

  4. Each property or constructor argument which is a value must be able to be converted from whatever format it was specified in, to the actual type of that property or constructor argument. By default Spring can convert a value supplied in string format to all built-in types, such as int, long, String, boolean, etc.

The Spring container validates the configuration of each bean as the container is created, including the validation that properties which are bean references are actually referring to valid beans. However, the bean properties themselves are not set until the bean is actually created. For those beans that are singleton-scoped and set to be pre-instantiated (such as singleton beans in an ApplicationContext), creation happens at the time that the container is created, but otherwise this is only when the bean is requested. When a bean actually has to be created, this will potentially cause a graph of other beans to be created, as its dependencies and its dependencies' dependencies (and so on) are created and assigned.

You can generally trust Spring to do the right thing. It will detect misconfiguration issues, such as references to non-existent beans and circular dependencies, at container load-time. It will actually set properties and resolve dependencies as late as possible, which is when the bean is actually created. This means that a Spring container which has loaded correctly can later generate an exception when you request a bean if there is a problem creating that bean or one of its dependencies. This could happen if the bean throws an exception as a result of a missing or invalid property, for example. This potentially delayed visibility of some configuration issues is why ApplicationContext implementations by default pre-instantiate singleton beans. At the cost of some upfront time and memory to create these beans before they are actually needed, you find out about configuration issues when the ApplicationContext is created, not later. If you wish, you can still override this default behavior and set any of these singleton beans to lazy-initialize (that is not be pre-instantiated).

If no circular dependencies are involved (see sidebar for a discussion of circular dependencies), when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being passed (via one of the DI flavors) to the dependent bean. This means that if bean A has a dependency on bean B, the Spring IoC container will totally configure bean B prior to invoking the setter method on bean A; you can read 'totally configure' to mean that the bean will be instantiated (if not a pre-instantiated singleton), all of its dependencies will be set, and the relevant lifecycle methods (such as a configured init method or the IntializingBean callback method) will all be invoked.

4.3.1.3 Some examples

First, an example of using XML-based configuration metadata for setter-based DI. Find below a small part of a Spring XML configuration file specifying some bean definitions.

<bean id="exampleBean" class="examples.ExampleBean">

  <!-- setter injection using the nested <ref/> element -->
  <property name="beanOne"><ref bean="anotherExampleBean"/></property>

  <!-- setter injection using the neater 'ref' attribute -->
  <property name="beanTwo" ref="yetAnotherBean"/>
  <property name="integerProperty" value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {

    private AnotherBean beanOne;
    private YetAnotherBean beanTwo;
    private int i;

    public void setBeanOne(AnotherBean beanOne) {
        this.beanOne = beanOne;
    }

    public void setBeanTwo(YetAnotherBean beanTwo) {
        this.beanTwo = beanTwo;
    }

    public void setIntegerProperty(int i) {
        this.i = i;
    }    
}

As you can see, setters have been declared to match against the properties specified in the XML file. Find below an example of using constructor-based DI.

<bean id="exampleBean" class="examples.ExampleBean">

  <!-- constructor injection using the nested <ref/> element -->
  <constructor-arg>
    <ref bean="anotherExampleBean"/>
  </constructor-arg>
  
  <!-- constructor injection using the neater 'ref' attribute -->
  <constructor-arg ref="yetAnotherBean"/>
  
  <constructor-arg type="int" value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {

    private AnotherBean beanOne;
    private YetAnotherBean beanTwo;
    private int i;
    
    public ExampleBean(
        AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
        this.beanOne = anotherBean;
        this.beanTwo = yetAnotherBean;
        this.i = i;
    }
}

As you can see, the constructor arguments specified in the bean definition will be used to pass in as arguments to the constructor of the ExampleBean.

Now consider a variant of this where instead of using a constructor, Spring is told to call a static factory method to return an instance of the object:

<bean id="exampleBean" class="examples.ExampleBean"
      factory-method="createInstance">
  <constructor-arg ref="anotherExampleBean"/>
  <constructor-arg ref="yetAnotherBean"/>
  <constructor-arg value="1"/> 
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {

    // a private constructor
    private ExampleBean(...) {
      ...
    }
    
    // a static factory method; the arguments to this method can be
    // considered the dependencies of the bean that is returned,
    // regardless of how those arguments are actually used.
    public static ExampleBean createInstance (
            AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {

        ExampleBean eb = new ExampleBean (...);
        // some other operations...
        return eb;
    }
}

Note that arguments to the static factory method are supplied via <constructor-arg/> elements, exactly the same as if a constructor had actually been used. Also, it is important to realize that the type of the class being returned by the factory method does not have to be of the same type as the class which contains the static factory method, although in this example it is. An instance (non-static) factory method would be used in an essentially identical fashion (aside from the use of the factory-bean attribute instead of the class attribute), so details will not be discussed here.

4.3.2 Dependencies and configuration in detail

As mentioned in the previous section, bean properties and constructor arguments can be defined as either references to other managed beans (collaborators), or values defined inline. Spring's XML-based configuration metadata supports a number of sub-element types within its <property/> and <constructor-arg/> elements for just this purpose.

4.3.2.1 Straight values (primitives, Strings, etc.)

The <value/> element specifies a property or constructor argument as a human-readable string representation. As mentioned previously, JavaBeans PropertyEditors are used to convert these string values from a String to the actual type of the property or argument.

<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
  
  <!-- results in a setDriverClassName(String) call -->
  <property name="driverClassName">
    <value>com.mysql.jdbc.Driver</value>
  </property>
  <property name="url">
    <value>jdbc:mysql://localhost:3306/mydb</value>
  </property>
  <property name="username">
    <value>root</value>
  </property>
  <property name="password">
    <value>masterkaoli</value>
  </property>
</bean>

The <property/> and <constructor-arg/> elements also support the use of the 'value' attribute, which can lead to much more succinct configuration. When using the 'value' attribute, the above bean definition reads like so:

<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
  
  <!-- results in a setDriverClassName(String) call -->
  <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
  <property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
  <property name="username" value="root"/>
  <property name="password" value="masterkaoli"/>
</bean>

The Spring team generally prefer the attribute style over the use of nested <value/> elements. If you are reading this reference manual straight through from top to bottom (wow!) then we are getting slightly ahead of ourselves here, but you can also configure a java.util.Properties instance like so:

<bean id="mappings" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
            
   <!-- typed as a java.util.Properties -->
   <property name="properties">
      <value>
         jdbc.driver.className=com.mysql.jdbc.Driver
         jdbc.url=jdbc:mysql://localhost:3306/mydb
      </value>
   </property>
</bean>

Can you see what is happening? The Spring container is converting the text inside the <value/> element into a java.util.Properties instance using the JavaBeans PropertyEditor mechanism. This is a nice shortcut, and is one of a few places where the Spring team do favor the use of the nested <value/> element over the 'value' attribute style.

The idref element

The idref element is simply an error-proof way to pass the id of another bean in the container (to a <constructor-arg/> or <property/> element).

<bean id="theTargetBean" class="..."/>

<bean id="theClientBean" class="...">
    <property name="targetName">
        <idref bean="theTargetBean" />
    </property>
</bean>

The above bean definition snippet is exactly equivalent (at runtime) to the following snippet:

<bean id="theTargetBean" class="..." />

<bean id="client" class="...">
    <property name="targetName" value="theTargetBean" />
</bean>

The main reason the first form is preferable to the second is that using the idref tag allows the container to validate at deployment time that the referenced, named bean actually exists. In the second variation, no validation is performed on the value that is passed to the 'targetName' property of the 'client' bean. Any typo will only be discovered (with most likely fatal results) when the 'client' bean is actually instantiated. If the 'client' bean is a prototype bean, this typo (and the resulting exception) may only be discovered long after the container is actually deployed.

Additionally, if the bean being referred to is in the same XML unit, and the bean name is the bean id, the 'local' attribute may be used, which allows the XML parser itself to validate the bean id even earlier, at XML document parse time.

<property name="targetName">
   <!-- a bean with an id of 'theTargetBean' must exist; otherwise an XML exception will be thrown -->
   <idref local="theTargetBean"/>
</property>

By way of an example, one common place (at least in pre-Spring 2.0 configuration) where the <idref/> element brings value is in the configuration of AOP interceptors in a ProxyFactoryBean bean definition. If you use <idref/> elements when specifying the interceptor names, there is no chance of inadvertently misspelling an interceptor id.

4.3.2.2 References to other beans (collaborators)

The ref element is the final element allowed inside a <constructor-arg/> or <property/> definition element. It is used to set the value of the specified property to be a reference to another bean managed by the container (a collaborator). As mentioned in a previous section, the referred-to bean is considered to be a dependency of the bean who's property is being set, and will be initialized on demand as needed (if it is a singleton bean it may have already been initialized by the container) before the property is set. All references are ultimately just a reference to another object, but there are 3 variations on how the id/name of the other object may be specified, which determines how scoping and validation is handled.

Specifying the target bean by using the bean attribute of the <ref/> tag is the most general form, and will allow creating a reference to any bean in the same container (whether or not in the same XML file), or parent container. The value of the 'bean' attribute may be the same as either the 'id' attribute of the target bean, or one of the values in the 'name' attribute of the target bean.

<ref bean="someBean"/>

Specifying the target bean by using the local attribute leverages the ability of the XML parser to validate XML id references within the same file. The value of the local attribute must be the same as the id attribute of the target bean. The XML parser will issue an error if no matching element is found in the same file. As such, using the local variant is the best choice (in order to know about errors as early as possible) if the target bean is in the same XML file.

<ref local="someBean"/>

Specifying the target bean by using the 'parent' attribute allows a reference to be created to a bean which is in a parent container of the current container. The value of the 'parent' attribute may be the same as either the 'id' attribute of the target bean, or one of the values in the 'name' attribute of the target bean, and the target bean must be in a parent container to the current one. The main use of this bean reference variant is when you have a hierarchy of containers and you want to wrap an existing bean in a parent container with some sort of proxy which will have the same name as the parent bean.

<!-- in the parent context -->
<bean id="accountService" class="com.foo.SimpleAccountService">
    <!-- insert dependencies as required as here -->
</bean>
<!-- in the child (descendant) context -->
<bean id="accountService"  <-- notice that the name of this bean is the same as the name of the 'parent' bean
      class="org.springframework.aop.framework.ProxyFactoryBean">
      <property name="target">
          <ref parent="accountService"/>  <-- notice how we refer to the parent bean
      </property>
    <!-- insert other configuration and dependencies as required as here -->
</bean>

4.3.2.3 Inner beans

A <bean/> element inside the <property/> or <constructor-arg/> elements is used to define a so-called inner bean. An inner bean definition does not need to have any id or name defined, and it is best not to even specify any id or name value because the id or name value simply will be ignored by the container.

<bean id="outer" class="...">
  <!-- instead of using a reference to a target bean, simply define the target bean inline -->
  <property name="target">
    <bean class="com.example.Person"> <!-- this is the inner bean -->
      <property name="name" value="Fiona Apple"/>
      <property name="age" value="25"/>
    </bean>
  </property>
</bean>

Note that in the specific case of inner beans, the 'scope' flag and any 'id' or 'name' attribute are effectively ignored. Inner beans are always anonymous and they are always scoped as prototypes. Please also note that it is not possible to inject inner beans into collaborating beans other than the enclosing bean.

4.3.2.4 Collections

The <list/>, <set/>, <map/>, and <props/> elements allow properties and arguments of the Java Collection type List, Set, Map, and Properties, respectively, to be defined and set.

<bean id="moreComplexObject" class="example.ComplexObject">
  <!-- results in a setAdminEmails(java.util.Properties) call -->
  <property name="adminEmails">
    <props>
        <prop key="administrator">administrator@example.org</prop>
        <prop key="support">support@example.org</prop>
        <prop key="development">development@example.org</prop>
    </props>
  </property>
  <!-- results in a setSomeList(java.util.List) call -->
  <property name="someList">
    <list>
        <value>a list element followed by a reference</value>
        <ref bean="myDataSource" />
    </list>
  </property>
  <!-- results in a setSomeMap(java.util.Map) call -->
  <property name="someMap">
    <map>
        <entry>
            <key>
                <value>an entry</value>
            </key>
            <value>just some string</value>
        </entry>
        <entry>
            <key>
                <value>a ref</value>
            </key>
            <ref bean="myDataSource" />
        </entry>
    </map>
  </property>
  <!-- results in a setSomeSet(java.util.Set) call -->
  <property name="someSet">
    <set>
        <value>just some string</value>
        <ref bean="myDataSource" />
    </set>
  </property>
</bean>
[Note]Note

The nested element style used this initial example tends to become quite verbose. Fortunately, there are attribute shortcuts for most elements, which you can read about in Section 4.3.2.6, “Shortcuts and other convenience options for XML-based configuration metadata”.

Note that the value of a map key or value, or a set value, can also again be any of the following elements:

bean | ref | idref | list | set | map | props | value | null
Collection merging

As of Spring 2.0, the container also supports the merging of collections. This allows an application developer to define a parent-style <list/>, <map/>, <set/> or <props/> element, and have child-style <list/>, <map/>, <set/> or <props/> elements inherit and override values from the parent collection; that is to say the child collection's values will be the result obtained from the merging of the elements of the parent and child collections, with the child's collection elements overriding values specified in the parent collection.

Please note that this section on merging makes use of the parent-child bean mechanism. This concept has not yet been introduced, so readers unfamiliar with the concept of parent and child bean definitions may wish to read the relevant section before continuing.

Find below an example of the collection merging feature:

<beans>
<bean id="parent" abstract="true" class="example.ComplexObject">
    <property name="adminEmails">
        <props>
            <prop key="administrator">administrator@example.com</prop>
            <prop key="support">support@example.com</prop>
        </props>
    </property>
</bean>
<bean id="child" parent="parent">
    <property name="adminEmails">
        <!-- the merge is specified on the *child* collection definition -->
        <props merge="true">
            <prop key="sales">sales@example.com</prop>
            <prop key="support">support@example.co.uk</prop>
        </props>
    </property>
</bean>
<beans>

Notice the use of the merge=true attribute on the <props/> element of the adminEmails property of the child bean definition. When the child bean is actually resolved and instantiated by the container, the resulting instance will have an adminEmails Properties collection that contains the result of the merging of the child's adminEmails collection with the parent's adminEmails collection.

administrator=administrator@example.com
sales=sales@example.com
support=support@example.co.uk

Notice how the child Properties collection's value set will have inherited all the property elements from the parent <props/>. Notice also how the child's value for the support value overrides the value in the parent collection.

This merging behavior applies similarly to the <list/>, <map/>, and <set/> collection types. In the specific case of the <list/> element, the semantics associated with the List collection type, that is the notion of an ordered collection of values, is maintained; the parent's values will precede all of the child list's values. In the case of the Map, Set, and Properties collection types, there is no notion of ordering and hence no ordering semantics are in effect for the collection types that underlie the associated Map, Set and Properties implementation types used internally by the container.

Finally, some minor notes about the merging support are in order; you cannot merge different collection types (e.g. a Map and a List), and if you do attempt to do so an appropriate Exception will be thrown; and in case it is not immediately obvious, the 'merge' attribute must be specified on the lower level, inherited, child definition; specifying the 'merge' attribute on a parent collection definition is redundant and will not result in the desired merging; and (lastly), please note that this merging feature is only available in Spring 2.0 (and later versions).

Strongly-typed collection (Java 5+ only)

If you are using Java 5 or Java 6, you will be aware that it is possible to have strongly typed collections (using generic types). That is, it is possible to declare a Collection type such that it can only contain String elements (for example). If you are using Spring to dependency inject a strongly-typed Collection into a bean, you can take advantage of Spring's type-conversion support such that the elements of your strongly-typed Collection instances will be converted to the appropriate type prior to being added to the Collection.

public class Foo {
                
    private Map<String, Float> accounts;
    
    public void setAccounts(Map<String, Float> accounts) {
        this.accounts = accounts;
    }
}
<beans>
    <bean id="foo" class="x.y.Foo">
        <property name="accounts">
            <map>
                <entry key="one" value="9.99"/>
                <entry key="two" value="2.75"/>
                <entry key="six" value="3.99"/>
            </map>
        </property>
    </bean>
</beans>

When the 'accounts' property of the 'foo' bean is being prepared for injection, the generics information about the element type of the strongly-typed Map<String, Float> is actually available via reflection, and so Spring's type conversion infrastructure will actually recognize the various value elements as being of type Float and so the string values '9.99', '2.75', and '3.99' will be converted into an actual Float type.

4.3.2.5 Nulls

The <null/> element is used to handle null values. Spring treats empty arguments for properties and the like as empty Strings. The following XML-based configuration metadata snippet results in the email property being set to the empty String value ("")

<bean class="ExampleBean">
  <property name="email"><value/></property>
</bean>

This is equivalent to the following Java code: exampleBean.setEmail(""). The special <null> element may be used to indicate a null value. For example:

<bean class="ExampleBean">
  <property name="email"><null/></property>
</bean>

The above configuration is equivalent to the following Java code: exampleBean.setEmail(null).

4.3.2.6 Shortcuts and other convenience options for XML-based configuration metadata

The configuration metadata shown so far is a tad verbose. That is why there are several options available for you to limit the amount of XML you have to write to configure your components. The first is a shortcut to define values and references to other beans as part of a <property/> definition. The second is slightly different format of specifying properties altogether.

XML-based configuration metadata shortcuts

The <property/>, <constructor-arg/>, and <entry/> elements all support a 'value' attribute which may be used instead of embedding a full <value/> element. Therefore, the following:

<property name="myProperty">
  <value>hello</value>
</property>
<constructor-arg>
  <value>hello</value>
</constructor-arg>
<entry key="myKey">
  <value>hello</value>
</entry>

are equivalent to:

<property name="myProperty" value="hello"/>
<constructor-arg value="hello"/>
<entry key="myKey" value="hello"/>

The <property/> and <constructor-arg/> elements support a similar shortcut 'ref' attribute which may be used instead of a full nested <ref/> element. Therefore, the following:

<property name="myProperty">
  <ref bean="myBean">
</property>
<constructor-arg>
  <ref bean="myBean">
</constructor-arg>

... are equivalent to:

<property name="myProperty" ref="myBean"/>
<constructor-arg ref="myBean"/>

Note however that the shortcut form is equivalent to a <ref bean="xxx"> element; there is no shortcut for <ref local="xxx">. To enforce a strict local reference, you must use the long form.

Finally, the entry element allows a shortcut form to specify the key and/or value of the map, in the form of the 'key' / 'key-ref' and 'value' / 'value-ref' attributes. Therefore, the following:

<entry>
  <key>
    <ref bean="myKeyBean" />
  </key>
  <ref bean="myValueBean" />
</entry>

is equivalent to:

<entry key-ref="myKeyBean" value-ref="myValueBean"/>

Again, the shortcut form is equivalent to a <ref bean="xxx"> element; there is no shortcut for <ref local="xxx">.

The p-namespace and how to use it to configure properties

The second option you have to limit the amount of XML you have to write to configure your components is to use the special "p-namespace". Spring 2.0 and later features support for extensible configuration formats using namespaces. Those namespaces are all based on an XML Schema definition. In fact, the beans configuration format that you've been reading about is defined in an XML Schema document.

One special namespace is not defined in an XSD file, and only exists in the core of Spring itself. The so-called p-namespace doesn't need a schema definition and is an alternative way of configuring your properties differently than the way you have seen so far. Instead of using nested <property/> elements, using the p-namespace you can use attributes as part of the bean element that describe your property values. The values of the attributes will be taken as the values for your properties.

The following two XML snippets boil down to the same thing in the end: the first is using the standard XML format whereas the second example is using the p-namespace.

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    
    <bean name="classic" class="com.example.ExampleBean">
        <property name="email" value="foo@bar.com/>
    </bean>
    
    <bean name="p-namespace" class="com.example.ExampleBean"
          p:email="foo@bar.com"/>
</beans>

As you can see, we are including an attribute in the p-namespace called email in the bean definition - this is telling Spring that it should include a property declaration. As previously mentioned, the p-namespace doesn't have a schema definition, so the name of the attribute can be set to whatever name your property has.

This next example includes two more bean definitions that both have a reference to another bean:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    
    <bean name="john-classic" class="com.example.Person">
        <property name="name" value="John Doe"/>
        <property name="spouse" ref="jane"/>
    </bean>

    <bean name="john-modern" 
        class="com.example.Person"
        p:name="John Doe"
        p:spouse-ref="jane"/>

    <bean name="jane" class="com.example.Person">
        <property name="name" value="Jane Doe"/>
    </bean>
</beans>

As you can see, this example doesn't only include a property value using the p-namespace, but also uses a special format to declare property references. Whereas the first bean definition uses <property name="spouse" ref="jane"/> to create a reference from bean john to bean jane, the second bean definition uses p:spouse-ref="jane" as an attribute to do the exact same thing. In this case 'spouse' is the property name whereas the '-ref' part indicates that this is not a straight value but rather a reference to another bean.

[Note]Note

Please note that the p-namespace is not quite as flexible as the standard XML format - for example particular, the 'special' format used to declare property references will clash with properties that end in 'Ref', whereas the standard XML format would have no problem there. We recommend that you choose carefully which approach you are going to use in your projects. You should also communicate this to your team members so you won't end up with XML documents using all three approaches at the same time. This will prevent people from not understanding the application because of different ways of configuring it, and will add to the overall consistency of your codebase.

4.3.2.7 Compound property names

Compound or nested property names are perfectly legal when setting bean properties, as long as all components of the path except the final property name are not null. Consider the following bean definition...

<bean id="foo" class="foo.Bar">
  <property name="fred.bob.sammy" value="123" />
</bean>

The foo bean has a fred property which has a bob property, which has a sammy property, and that final sammy property is being set to the value 123. In order for this to work, the fred property of foo, and the bob property of fred must not be null be non-null after the bean is constructed, or a NullPointerException will be thrown.

4.3.3 Using depends-on

For most situations, the fact that a bean is a dependency of another is expressed by the fact that one bean is set as a property of another. This is typically accomplished with the <ref/> element in XML-based configuration metadata. For the relatively infrequent situations where dependencies between beans are less direct (for example, when a static initializer in a class needs to be triggered, such as database driver registration), the 'depends-on' attribute may be used to explicitly force one or more beans to be initialized before the bean using this element is initialized. Find below an example of using the 'depends-on' attribute to express a dependency on a single bean.

<bean id="beanOne" class="ExampleBean" depends-on="manager"/>

<bean id="manager" class="ManagerBean" />

If you need to express a dependency on multiple beans, you can supply a list of bean names as the value of the 'depends-on' attribute, with commas, whitespace and semicolons all valid delimiters, like so:

<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao">
  <property name="manager" ref="manager" />
</bean>

<bean id="manager" class="ManagerBean" />
<bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
[Note]Note

The 'depends-on' attribute at the bean definition level is used not only to specify an initialization time dependency, but also to specify the corresponding destroy time dependency (in the case of singleton beans only). Dependent beans that define a 'depends-on' relationship with a given bean will be destroyed first - prior to the given bean itself being destroyed. As a consequence, 'depends-on' may be used to control shutdown order too.

4.3.4 Lazily-instantiated beans

The default behavior for ApplicationContext implementations is to eagerly pre-instantiate all singleton beans at startup. Pre-instantiation means that an ApplicationContext will eagerly create and configure all of its singleton beans as part of its initialization process. Generally this is a good thing, because it means that any errors in the configuration or in the surrounding environment will be discovered immediately (as opposed to possibly hours or even days down the line).

However, there are times when this behavior is not what is wanted. If you do not want a singleton bean to be pre-instantiated when using an ApplicationContext, you can selectively control this by marking a bean definition as lazy-initialized. A lazily-initialized bean indicates to the IoC container whether or not a bean instance should be created at startup or when it is first requested.

When configuring beans via XML, this lazy loading is controlled by the 'lazy-init' attribute on the <bean/> element; for example:

<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/>

<bean name="not.lazy" class="com.foo.AnotherBean"/>

When the above configuration is consumed by an ApplicationContext, the bean named 'lazy' will not be eagerly pre-instantiated when the ApplicationContext is starting up, whereas the 'not.lazy' bean will be eagerly pre-instantiated.

One thing to understand about lazy-initialization is that even though a bean definition may be marked up as being lazy-initialized, if the lazy-initialized bean is the dependency of a singleton bean that is not lazy-initialized, when the ApplicationContext is eagerly pre-instantiating the singleton, it will have to satisfy all of the singletons dependencies, one of which will be the lazy-initialized bean! So don't be confused if the IoC container creates one of the beans that you have explicitly configured as lazy-initialized at startup; all that means is that the lazy-initialized bean is being injected into a non-lazy-initialized singleton bean elsewhere.

It is also possible to control lazy-initialization at the container level by using the 'default-lazy-init' attribute on the <beans/> element; for example:

<beans default-lazy-init="true">
    <!-- no beans will be pre-instantiated... -->
</beans>

4.3.5 Autowiring collaborators

The Spring container is able to autowire relationships between collaborating beans. This means that it is possible to automatically let Spring resolve collaborators (other beans) for your bean by inspecting the contents of the BeanFactory. The autowiring functionality has five modes. Autowiring is specified per bean and can thus be enabled for some beans, while other beans will not be autowired. Using autowiring, it is possible to reduce or eliminate the need to specify properties or constructor arguments, thus saving a significant amount of typing. [2] When using XML-based configuration metadata, the autowire mode for a bean definition is specified by using the autowire attribute of the <bean/> element. The following values are allowed:

Table 4.2. Autowiring modes

ModeExplanation
no

No autowiring at all. Bean references must be defined via a ref element. This is the default, and changing this is discouraged for larger deployments, since explicitly specifying collaborators gives greater control and clarity. To some extent, it is a form of documentation about the structure of a system.

byName

Autowiring by property name. This option will inspect the container and look for a bean named exactly the same as the property which needs to be autowired. For example, if you have a bean definition which is set to autowire by name, and it contains a master property (that is, it has a setMaster(..) method), Spring will look for a bean definition named master, and use it to set the property.

byType

Allows a property to be autowired if there is exactly one bean of the property type in the container. If there is more than one, a fatal exception is thrown, and this indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set. If this is not desirable, setting the dependency-check="objects" attribute value specifies that an error should be thrown in this case.

constructor

This is analogous to byType, but applies to constructor arguments. If there isn't exactly one bean of the constructor argument type in the container, a fatal error is raised.

autodetect

Chooses constructor or byType through introspection of the bean class. If a default constructor is found, the byType mode will be applied.


Note that explicit dependencies in property and constructor-arg settings always override autowiring. Please also note that it is not currently possible to autowire so-called simple properties such as primitives, Strings, and Classes (and arrays of such simple properties). (This is by-design and should be considered a feature.) When using either the byType or constructor autowiring mode, it is possible to wire arrays and typed-collections. In such cases all autowire candidates within the container that match the expected type will be provided to satisfy the dependency. Strongly-typed Maps can even be autowired if the expected key type is String. An autowired Map's values will consist of all bean instances that match the expected type, and the Map's keys will contain the corresponding bean names.

Autowire behavior can be combined with dependency checking, which will be performed after all autowiring has been completed.

It is important to understand the various advantages and disadvantages of autowiring. Some advantages of autowiring include:

  • Autowiring can significantly reduce the volume of configuration required. However, mechanisms such as the use of a bean template (discussed elsewhere in this chapter) are also valuable in this regard.

  • Autowiring can cause configuration to keep itself up to date as your objects evolve. For example, if you need to add an additional dependency to a class, that dependency can be satisfied automatically without the need to modify configuration. Thus there may be a strong case for autowiring during development, without ruling out the option of switching to explicit wiring when the code base becomes more stable.

Some disadvantages of autowiring:

  • Autowiring is more magical than explicit wiring. Although, as noted in the above table, Spring is careful to avoid guessing in case of ambiguity which might have unexpected results, the relationships between your Spring-managed objects are no longer documented explicitly.

  • Wiring information may not be available to tools that may generate documentation from a Spring container.

Another issue to consider when autowiring by type is that multiple bean definitions within the container may match the type specified by the setter method or constructor argument to be autowired. For arrays, collections, or Maps, this is not necessarily a problem. However for dependencies that expect a single value, this ambiguity will not be arbitrarily resolved. Instead, if no unique bean definition is available, an Exception will be thrown. You do have several options when confronted with this scenario. First, you may abandon autowiring in favor of explicit wiring. Second, you may designate that certain bean definitions are never to be considered as candidates by setting their 'autowire-candidate' attributes to 'false' as described in the next section. Third, you may designate a single bean definition as the primary candidate by setting the 'primary' attribute of its <bean/> element to 'true'. Finally, if you are using at least Java 5, you may be interested in exploring the more fine-grained control available with annotation-based configuration as described in the section entitled Section 4.11, “Annotation-based configuration”.

When deciding whether to use autowiring, there is no wrong or right answer in all cases. A degree of consistency across a project is best though; for example, if autowiring is not used in general, it might be confusing to developers to use it just to wire one or two bean definitions.

4.3.5.1 Excluding a bean from being available for autowiring

You can also (on a per-bean basis) totally exclude a bean from being an autowire candidate. When configuring beans using Spring's XML format, the 'autowire-candidate' attribute of the <bean/> element can be set to 'false'; this has the effect of making the container totally exclude that specific bean definition from being available to the autowiring infrastructure.

Another option is to limit autowire candidates based on pattern-matching against bean names. The top-level <beans/> element accepts one or more patterns within its 'default-autowire-candidates' attribute. For example, to limit autowire candidate status to any bean whose name ends with 'Repository', provide a value of '*Repository'. To provide multiple patterns, define them in a comma-separated list. Note that an explicit value of 'true' or 'false' for a bean definition's 'autowire-candidate' attribute always takes precedence, and for such beans, the pattern matching rules will not apply.

These techniques can be useful when you have one or more beans that you absolutely never ever want to have injected into other beans via autowiring. It does not mean that an excluded bean cannot itself be configured using autowiring... it can, it is rather that it itself will not be considered as a candidate for autowiring other beans.

4.3.6 Checking for dependencies

The Spring IoC container also has the ability to check for the existence of unresolved dependencies of a bean deployed into the container. These are JavaBeans properties of the bean, which do not have actual values set for them in the bean definition, or alternately provided automatically by the autowiring feature.

This feature is sometimes useful when you want to ensure that all properties (or all properties of a certain type) are set on a bean. Of course, in many cases a bean class will have default values for many properties, or some properties do not apply to all usage scenarios, so this feature is of limited use. Dependency checking can also be enabled and disabled per bean, just as with the autowiring functionality. The default is to not check dependencies. Dependency checking can be handled in several different modes. When using XML-based configuration metadata, this is specified via the 'dependency-check' attribute in a bean definition, which may have the following values.

Table 4.3. Dependency checking modes

ModeExplanation
none

No dependency checking. Properties of the bean which have no value specified for them are simply not set.

simple

Dependency checking is performed for primitive types and collections (everything except collaborators).

object

Dependency checking is performed for collaborators only.

all

Dependency checking is done for collaborators, primitive types and collections.


If you are using Java 5 and thus have access to source-level annotations, you may find the section entitled Section 29.3.1, “@Required” to be of interest.

4.3.7 Method Injection

For most application scenarios, the majority of the beans in the container will be singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, the typical and common approach of handling this dependency by defining one bean to be a property of the other is quite adequate. There is a problem when the bean lifecycles are different. Consider a singleton bean A which needs to use a non-singleton (prototype) bean B, perhaps on each method invocation on A. The container will only create the singleton bean A once, and thus only get the opportunity to set the properties once. There is no opportunity for the container to provide bean A with a new instance of bean B every time one is needed.

One solution to this issue is to forego some inversion of control. Bean A can be made aware of the container by implementing the BeanFactoryAware interface, and use programmatic means to ask the container via a getBean("B") call for (a typically new) bean B instance every time it needs it. Find below an admittedly somewhat contrived example of this approach:

// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;

// lots of Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;

public class CommandManager implements BeanFactoryAware {

   private BeanFactory beanFactory;

   public Object process(Map commandState) {
      // grab a new instance of the appropriate Command
      Command command = createCommand();
      // set the state on the (hopefully brand new) Command instance
      command.setState(commandState);
      return command.execute();
   }

   // the Command returned here could be an implementation that executes asynchronously, or whatever
   protected Command createCommand() {
      return (Command) this.beanFactory.getBean("command"); // notice the Spring API dependency
   }

   public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
      this.beanFactory = beanFactory;
   }
}

The above example is generally not a desirable solution since the business code is then aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, allows this use case to be handled in a clean fashion.

4.3.7.1 Lookup method injection

Lookup method injection refers to the ability of the container to override methods on container managed beans, to return the result of looking up another named bean in the container. The lookup will typically be of a prototype bean as in the scenario described above. The Spring Framework implements this method injection by dynamically generating a subclass overriding the method, using bytecode generation via the CGLIB library.

So if you look at the code from previous code snippet (the CommandManager class), the Spring container is going to dynamically override the implementation of the createCommand() method. Your CommandManager class is not going to have any Spring dependencies, as can be seen in this reworked example below:

package fiona.apple;

// no more Spring imports! 

public abstract class CommandManager {

   public Object process(Object commandState) {
      // grab a new instance of the appropriate Command interface
      Command command = createCommand();
      // set the state on the (hopefully brand new) Command instance
      command.setState(commandState);
      return command.execute();
   }

    // okay... but where is the implementation of this method?
   protected abstract Command createCommand();
}

In the client class containing the method to be injected (the CommandManager in this case), the method that is to be 'injected' must have a signature of the following form:

<public|protected> [abstract] <return-type> theMethodName(no-arguments);

If the method is abstract, the dynamically-generated subclass will implement the method. Otherwise, the dynamically-generated subclass will override the concrete method defined in the original class. Let's look at an example:

<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="command" class="fiona.apple.AsyncCommand" scope="prototype">
  <!-- inject dependencies here as required -->
</bean>

<!-- commandProcessor uses statefulCommandHelper -->
<bean id="commandManager" class="fiona.apple.CommandManager">
  <lookup-method name="createCommand" bean="command"/>
</bean>

The bean identified as commandManager will call its own method createCommand() whenever it needs a new instance of the command bean. It is important to note that the person deploying the beans must be careful to deploy the command bean as a prototype (if that is actually what is needed). If it is deployed as a singleton, the same instance of the command bean will be returned each time!

Please be aware that in order for this dynamic subclassing to work, you will need to have the CGLIB jar(s) on your classpath. Additionally, the class that the Spring container is going to subclass cannot be final, and the method that is being overridden cannot be final either. Also, testing a class that has an abstract method can be somewhat odd in that you will have to subclass the class yourself and supply a stub implementation of the abstract method. Finally, objects that have been the target of method injection cannot be serialized.

[Tip]Tip

The interested reader may also find the ServiceLocatorFactoryBean (in the org.springframework.beans.factory.config package) to be of use; the approach is similar to that of the ObjectFactoryCreatingFactoryBean, but it allows you to specify your own lookup interface as opposed to having to use a Spring-specific lookup interface such as the ObjectFactory. Consult the (copious) Javadoc for the ServiceLocatorFactoryBean for a full treatment of this alternative approach (that does reduce the coupling to Spring).

4.3.7.2 Arbitrary method replacement

A less commonly useful form of method injection than Lookup Method Injection is the ability to replace arbitrary methods in a managed bean with another method implementation. Users may safely skip the rest of this section (which describes this somewhat advanced feature), until this functionality is actually needed.

When using XML-based configuration metadata, the replaced-method element may be used to replace an existing method implementation with another, for a deployed bean. Consider the following class, with a method computeValue, which we want to override:

public class MyValueCalculator {

  public String computeValue(String input) {
    // some real code...
  }

  // some other methods...

}

A class implementing the org.springframework.beans.factory.support.MethodReplacer interface provides the new method definition.

/** meant to be used to override the existing computeValue(String)
    implementation in MyValueCalculator
  */
public class ReplacementComputeValue implements MethodReplacer {

    public Object reimplement(Object o, Method m, Object[] args) throws Throwable {
        // get the input value, work with it, and return a computed result
        String input = (String) args[0];
        ... 
        return ...;
    }
}

The bean definition to deploy the original class and specify the method override would look like this:

<bean id="myValueCalculator class="x.y.z.MyValueCalculator">
  <!-- arbitrary method replacement -->
  <replaced-method name="computeValue" replacer="replacementComputeValue">
    <arg-type>String</arg-type>
  </replaced-method>
</bean>

<bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>

One or more contained <arg-type/> elements within the <replaced-method/> element may be used to indicate the method signature of the method being overridden. Note that the signature for the arguments is actually only needed in the case that the method is actually overloaded and there are multiple variants within the class. For convenience, the type string for an argument may be a substring of the fully qualified type name. For example, all the following would match java.lang.String.

    java.lang.String
    String
    Str

Since the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by allowing you to type just the shortest string that will match an argument type.