spring-boot-loader modules lets Spring Boot support executable jar and war files.
If you use the Maven plugin or the Gradle plugin, executable jars are automatically generated, and you generally do not need to know the details of how they work.
If you need to create executable jars from a different build system or if you are just curious about the underlying technology, this appendix provides some background.
1. Nested JARs
Java does not provide any standard way to load nested jar files (that is, jar files that are themselves contained within a jar). This can be problematic if you need to distribute a self-contained application that can be run from the command line without unpacking.
To solve this problem, many developers use “shaded” jars. A shaded jar packages all classes, from all jars, into a single “uber jar”. The problem with shaded jars is that it becomes hard to see which libraries are actually in your application. It can also be problematic if the same filename is used (but with different content) in multiple jars. Spring Boot takes a different approach and lets you actually nest jars directly.
1.1. The Executable Jar File Structure
Spring Boot Loader-compatible jar files should be structured in the following way:
example.jar | +-META-INF | +-MANIFEST.MF +-org | +-springframework | +-boot | +-loader | +-<spring boot loader classes> +-BOOT-INF +-classes | +-mycompany | +-project | +-YourClasses.class +-lib +-dependency1.jar +-dependency2.jar
Application classes should be placed in a nested
Dependencies should be placed in a nested
1.2. The Executable War File Structure
Spring Boot Loader-compatible war files should be structured in the following way:
example.war | +-META-INF | +-MANIFEST.MF +-org | +-springframework | +-boot | +-loader | +-<spring boot loader classes> +-WEB-INF +-classes | +-com | +-mycompany | +-project | +-YourClasses.class +-lib | +-dependency1.jar | +-dependency2.jar +-lib-provided +-servlet-api.jar +-dependency3.jar
Dependencies should be placed in a nested
Any dependencies that are required when running embedded but are not required when deploying to a traditional web container should be placed in
1.3. Index Files
Spring Boot Loader-compatible jar and war archives can include additional index files under the
classpath.idx file can be provided for both jars and wars, and it provides the ordering that jars should be added to the classpath.
layers.idx file can be used only for jars, and it allows a jar to be split into logical layers for Docker/OCI image creation.
Index files follow a YAML compatible syntax so that they can be easily parsed by third-party tools. These files, however, are not parsed internally as YAML and they must be written in exactly the formats described below in order to be used.
1.4. Classpath Index
The classpath index file can be provided in
It provides a list of jar names (including the directory) in the order that they should be added to the classpath.
Each line must start with dash space (
"-·") and names must be in double quotes.
For example, given the following jar:
example.jar | +-META-INF | +-... +-BOOT-INF +-classes | +... +-lib +-dependency1.jar +-dependency2.jar
The index file would look like this:
- "BOOT-INF/lib/dependency2.jar" - "BOOT-INF/lib/dependency1.jar"
1.5. Layer Index
The layers index file can be provided in
It provides a list of layers and the parts of the jar that should be contained within them.
Layers are written in the order that they should be added to the Docker/OCI image.
Layers names are written as quoted strings prefixed with dash space (
"-·") and with a colon (
Layer content is either a file or directory name written as a quoted string prefixed by space space dash space (
A directory name ends with
/, a file name does not.
When a directory name is used it means that all files inside that directory are in the same layer.
A typical example of a layers index would be:
- "dependencies": - "BOOT-INF/lib/dependency1.jar" - "BOOT-INF/lib/dependency2.jar" - "application": - "BOOT-INF/classes/" - "META-INF/"
2. Spring Boot’s “JarFile” Class
The core class used to support loading nested jars is
It lets you load jar content from a standard jar file or from nested child jar data.
When first loaded, the location of each
JarEntry is mapped to a physical file offset of the outer jar, as shown in the following example:
myapp.jar +-------------------+-------------------------+ | /BOOT-INF/classes | /BOOT-INF/lib/mylib.jar | |+-----------------+||+-----------+----------+| || A.class ||| B.class | C.class || |+-----------------+||+-----------+----------+| +-------------------+-------------------------+ ^ ^ ^ 0063 3452 3980
The preceding example shows how
A.class can be found in
myapp.jar at position
B.class from the nested jar can actually be found in
myapp.jar at position
C.class is at position
Armed with this information, we can load specific nested entries by seeking to the appropriate part of the outer jar. We do not need to unpack the archive, and we do not need to read all entry data into memory.
2.1. Compatibility With the Standard Java “JarFile”
Spring Boot Loader strives to remain compatible with existing code and libraries.
org.springframework.boot.loader.jar.JarFile extends from
java.util.jar.JarFile and should work as a drop-in replacement.
getURL() method returns a
URL that opens a connection compatible with
java.net.JarURLConnection and can be used with Java’s
3. Launching Executable Jars
org.springframework.boot.loader.Launcher class is a special bootstrap class that is used as an executable jar’s main entry point.
It is the actual
Main-Class in your jar file, and it is used to setup an appropriate
URLClassLoader and ultimately call your
There are three launcher subclasses (
Their purpose is to load resources (
.class files and so on) from nested jar files or war files in directories (as opposed to those explicitly on the classpath).
In the case of
WarLauncher, the nested paths are fixed.
JarLauncher looks in
WarLauncher looks in
You can add extra jars in those locations if you want more.
PropertiesLauncher looks in
BOOT-INF/lib/ in your application archive by default.
You can add additional locations by setting an environment variable called
loader.properties (which is a comma-separated list of directories, archives, or directories within archives).
3.1. Launcher Manifest
You need to specify an appropriate
Launcher as the
Main-Class attribute of
The actual class that you want to launch (that is, the class that contains a
main method) should be specified in the
The following example shows a typical
MANIFEST.MF for an executable jar file:
Main-Class: org.springframework.boot.loader.JarLauncher Start-Class: com.mycompany.project.MyApplication
For a war file, it would be as follows:
Main-Class: org.springframework.boot.loader.WarLauncher Start-Class: com.mycompany.project.MyApplication
You need not specify
4. PropertiesLauncher Features
PropertiesLauncher has a few special features that can be enabled with external properties (System properties, environment variables, manifest entries, or
The following table describes these properties:
Comma-separated Classpath, such as
Used to resolve relative paths in
Default arguments for the main method (space separated).
Name of main class to launch (for example,
Name of properties file (for example,
Path to properties file (for example,
Boolean flag to indicate that all properties should be added to System properties.
It defaults to
When specified as environment variables or manifest entries, the following names should be used:
|Key||Manifest entry||Environment variable|
Build plugins automatically move the
The following rules apply to working with
loader.propertiesis searched for in
loader.home, then in the root of the classpath, and then in
classpath:/BOOT-INF/classes. The first location where a file with that name exists is used.
loader.homeis the directory location of an additional properties file (overriding the default) only when
loader.config.locationis not specified.
loader.pathcan contain directories (which are scanned recursively for jar and zip files), archive paths, a directory within an archive that is scanned for jar files (for example,
dependencies.jar!/lib), or wildcard patterns (for the default JVM behavior). Archive paths can be relative to
loader.homeor anywhere in the file system with a
loader.path(if empty) defaults to
BOOT-INF/lib(meaning a local directory or a nested one if running from an archive). Because of this,
PropertiesLauncherbehaves the same as
JarLauncherwhen no additional configuration is provided.
loader.pathcan not be used to configure the location of
loader.properties(the classpath used to search for the latter is the JVM classpath when
Placeholder replacement is done from System and environment variables plus the properties file itself on all values before use.
The search order for properties (where it makes sense to look in more than one place) is environment variables, system properties,
loader.properties, the exploded archive manifest, and the archive manifest.
5. Executable Jar Restrictions
You need to consider the following restrictions when working with a Spring Boot Loader packaged application:
Zip entry compression: The
ZipEntryfor a nested jar must be saved by using the
ZipEntry.STOREDmethod. This is required so that we can seek directly to individual content within the nested jar. The content of the nested jar file itself can still be compressed, as can any other entries in the outer jar.
System classLoader: Launched applications should use
Thread.getContextClassLoader()when loading classes (most libraries and frameworks do so by default). Trying to load nested jar classes with
java.util.Loggingalways uses the system classloader. For this reason, you should consider a different logging implementation.
6. Alternative Single Jar Solutions
If the preceding restrictions mean that you cannot use Spring Boot Loader, consider the following alternatives: