`FlatFileItemWriter`

Writing out to flat files has the same problems and issues that reading in from a file must overcome. A step must be able to write either delimited or fixed length formats in a transactional manner.

`LineAggregator`

Just as the LineTokenizer interface is necessary to take an item and turn it into a String, file writing must have a way to aggregate multiple fields into a single string for writing to a file. In Spring Batch, this is the LineAggregator, shown in the following interface definition:

public interface LineAggregator<T> {

    public String aggregate(T item);

}

The LineAggregator is the logical opposite of LineTokenizer. LineTokenizer takes a String and returns a FieldSet, whereas LineAggregator takes an item and returns a String.

`PassThroughLineAggregator`

The most basic implementation of the LineAggregator interface is the PassThroughLineAggregator, which assumes that the object is already a string or that its string representation is acceptable for writing, as shown in the following code:

public class PassThroughLineAggregator<T> implements LineAggregator<T> {

    public String aggregate(T item) {
        return item.toString();
    }
}

The preceding implementation is useful if direct control of creating the string is required but the advantages of a FlatFileItemWriter, such as transaction and restart support, are necessary.

Simplified File Writing Example

Now that the LineAggregator interface and its most basic implementation, PassThroughLineAggregator, have been defined, the basic flow of writing can be explained:

The object to be written is passed to the LineAggregator in order to obtain a String.
The returned String is written to the configured file.

The following excerpt from the FlatFileItemWriter expresses this in code:

public void write(T item) throws Exception {
    write(lineAggregator.aggregate(item) + LINE_SEPARATOR);
}

Java
XML

In Java, a simple example of configuration might look like the following:

Java Configuration

@Bean
public FlatFileItemWriter itemWriter() {
	return  new FlatFileItemWriterBuilder<Foo>()
           			.name("itemWriter")
           			.resource(new FileSystemResource("target/test-outputs/output.txt"))
           			.lineAggregator(new PassThroughLineAggregator<>())
           			.build();
}

In XML, a simple example of configuration might look like the following:

XML Configuration

<bean id="itemWriter" class="org.spr...FlatFileItemWriter">
    <property name="resource" value="file:target/test-outputs/output.txt" />
    <property name="lineAggregator">
        <bean class="org.spr...PassThroughLineAggregator"/>
    </property>
</bean>

`FieldExtractor`

The preceding example may be useful for the most basic uses of a writing to a file. However, most users of the FlatFileItemWriter have a domain object that needs to be written out and, thus, must be converted into a line. In file reading, the following was required:

Read one line from the file.
Pass the line into the LineTokenizer#tokenize() method, in order to retrieve a FieldSet.
Pass the FieldSet returned from tokenizing to a FieldSetMapper, returning the result from the ItemReader#read() method.

File writing has similar but inverse steps:

Pass the item to be written to the writer.
Convert the fields on the item into an array.
Aggregate the resulting array into a line.

Because there is no way for the framework to know which fields from the object need to be written out, a FieldExtractor must be written to accomplish the task of turning the item into an array, as shown in the following interface definition:

public interface FieldExtractor<T> {

    Object[] extract(T item);

}

Implementations of the FieldExtractor interface should create an array from the fields of the provided object, which can then be written out with a delimiter between the elements or as part of a fixed-width line.

`PassThroughFieldExtractor`

There are many cases where a collection, such as an array, Collection, or FieldSet, needs to be written out. "Extracting" an array from one of these collection types is very straightforward. To do so, convert the collection to an array. Therefore, the PassThroughFieldExtractor should be used in this scenario. It should be noted that, if the object passed in is not a type of collection, then the PassThroughFieldExtractor returns an array containing solely the item to be extracted.

`BeanWrapperFieldExtractor`

As with the BeanWrapperFieldSetMapper described in the file reading section, it is often preferable to configure how to convert a domain object to an object array, rather than writing the conversion yourself. The BeanWrapperFieldExtractor provides this functionality, as shown in the following example:

BeanWrapperFieldExtractor<Name> extractor = new BeanWrapperFieldExtractor<>();
extractor.setNames(new String[] { "first", "last", "born" });

String first = "Alan";
String last = "Turing";
int born = 1912;

Name n = new Name(first, last, born);
Object[] values = extractor.extract(n);

assertEquals(first, values[0]);
assertEquals(last, values[1]);
assertEquals(born, values[2]);

This extractor implementation has only one required property: the names of the fields to map. Just as the BeanWrapperFieldSetMapper needs field names to map fields on the FieldSet to setters on the provided object, the BeanWrapperFieldExtractor needs names to map to getters for creating an object array. It is worth noting that the order of the names determines the order of the fields within the array.

Delimited File Writing Example

The most basic flat file format is one in which all fields are separated by a delimiter. This can be accomplished using a DelimitedLineAggregator. The following example writes out a simple domain object that represents a credit to a customer account:

public class CustomerCredit {

    private int id;
    private String name;
    private BigDecimal credit;

    //getters and setters removed for clarity
}

Because a domain object is being used, an implementation of the FieldExtractor interface must be provided, along with the delimiter to use.

Java
XML

The following example shows how to use the FieldExtractor with a delimiter in Java:

Java Configuration

@Bean
public FlatFileItemWriter<CustomerCredit> itemWriter(Resource outputResource) throws Exception {
	BeanWrapperFieldExtractor<CustomerCredit> fieldExtractor = new BeanWrapperFieldExtractor<>();
	fieldExtractor.setNames(new String[] {"name", "credit"});
	fieldExtractor.afterPropertiesSet();

	DelimitedLineAggregator<CustomerCredit> lineAggregator = new DelimitedLineAggregator<>();
	lineAggregator.setDelimiter(",");
	lineAggregator.setFieldExtractor(fieldExtractor);

	return new FlatFileItemWriterBuilder<CustomerCredit>()
				.name("customerCreditWriter")
				.resource(outputResource)
				.lineAggregator(lineAggregator)
				.build();
}

The following example shows how to use the FieldExtractor with a delimiter in XML:

XML Configuration

<bean id="itemWriter" class="org.springframework.batch.item.file.FlatFileItemWriter">
    <property name="resource" ref="outputResource" />
    <property name="lineAggregator">
        <bean class="org.spr...DelimitedLineAggregator">
            <property name="delimiter" value=","/>
            <property name="fieldExtractor">
                <bean class="org.spr...BeanWrapperFieldExtractor">
                    <property name="names" value="name,credit"/>
                </bean>
            </property>
        </bean>
    </property>
</bean>

In the previous example, the BeanWrapperFieldExtractor described earlier in this chapter is used to turn the name and credit fields within CustomerCredit into an object array, which is then written out with commas between each field.

Java
XML

It is also possible to use the FlatFileItemWriterBuilder.DelimitedBuilder to automatically create the BeanWrapperFieldExtractor and DelimitedLineAggregator as shown in the following example:

Java Configuration

@Bean
public FlatFileItemWriter<CustomerCredit> itemWriter(Resource outputResource) throws Exception {
	return new FlatFileItemWriterBuilder<CustomerCredit>()
				.name("customerCreditWriter")
				.resource(outputResource)
				.delimited()
				.delimiter("|")
				.names(new String[] {"name", "credit"})
				.build();
}

There is no XML equivalent of using FlatFileItemWriterBuilder.

Fixed Width File Writing Example

Delimited is not the only type of flat file format. Many prefer to use a set width for each column to delineate between fields, which is usually referred to as 'fixed width'. Spring Batch supports this in file writing with the FormatterLineAggregator.

Java
XML

Using the same CustomerCredit domain object described above, it can be configured as follows in Java:

Java Configuration

@Bean
public FlatFileItemWriter<CustomerCredit> itemWriter(Resource outputResource) throws Exception {
	BeanWrapperFieldExtractor<CustomerCredit> fieldExtractor = new BeanWrapperFieldExtractor<>();
	fieldExtractor.setNames(new String[] {"name", "credit"});
	fieldExtractor.afterPropertiesSet();

	FormatterLineAggregator<CustomerCredit> lineAggregator = new FormatterLineAggregator<>();
	lineAggregator.setFormat("%-9s%-2.0f");
	lineAggregator.setFieldExtractor(fieldExtractor);

	return new FlatFileItemWriterBuilder<CustomerCredit>()
				.name("customerCreditWriter")
				.resource(outputResource)
				.lineAggregator(lineAggregator)
				.build();
}

Using the same CustomerCredit domain object described above, it can be configured as follows in XML:

XML Configuration

<bean id="itemWriter" class="org.springframework.batch.item.file.FlatFileItemWriter">
    <property name="resource" ref="outputResource" />
    <property name="lineAggregator">
        <bean class="org.spr...FormatterLineAggregator">
            <property name="fieldExtractor">
                <bean class="org.spr...BeanWrapperFieldExtractor">
                    <property name="names" value="name,credit" />
                </bean>
            </property>
            <property name="format" value="%-9s%-2.0f" />
        </bean>
    </property>
</bean>

Most of the preceding example should look familiar. However, the value of the format property is new.

Java
XML

The following example shows the format property in Java:

...
FormatterLineAggregator<CustomerCredit> lineAggregator = new FormatterLineAggregator<>();
lineAggregator.setFormat("%-9s%-2.0f");
...

The following example shows the format property in XML:

<property name="format" value="%-9s%-2.0f" />

The underlying implementation is built using the same Formatter added as part of Java 5. The Java Formatter is based on the printf functionality of the C programming language. Most details on how to configure a formatter can be found in the Javadoc of Formatter.

Java
XML

It is also possible to use the FlatFileItemWriterBuilder.FormattedBuilder to automatically create the BeanWrapperFieldExtractor and FormatterLineAggregator as shown in following example:

Java Configuration

@Bean
public FlatFileItemWriter<CustomerCredit> itemWriter(Resource outputResource) throws Exception {
	return new FlatFileItemWriterBuilder<CustomerCredit>()
				.name("customerCreditWriter")
				.resource(outputResource)
				.formatted()
				.format("%-9s%-2.0f")
				.names(new String[] {"name", "credit"})
				.build();
}

Handling File Creation

FlatFileItemReader has a very simple relationship with file resources. When the reader is initialized, it opens the file (if it exists), and throws an exception if it does not. File writing isn’t quite so simple. At first glance, it seems like a similar straightforward contract should exist for FlatFileItemWriter: If the file already exists, throw an exception, and, if it does not, create it and start writing. However, potentially restarting a Job can cause issues. In normal restart scenarios, the contract is reversed: If the file exists, start writing to it from the last known good position, and, if it does not, throw an exception. However, what happens if the file name for this job is always the same? In this case, you would want to delete the file if it exists, unless it’s a restart. Because of this possibility, the FlatFileItemWriter contains the property, shouldDeleteIfExists. Setting this property to true causes an existing file with the same name to be deleted when the writer is opened.