Appendix E. The executable jar format

Appendix E. The executable jar format
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The spring-boot-loader modules allows Spring Boot to support executable jar and war files. If you’re using the Maven or Gradle plugin, executable jars are automatically generated and you generally won’t 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 section provides some background.

E.1 Nested JARs

Java does not provide any standard way to load nested jar files (i.e. jar files that are themselves contained within a jar). This can be problematic if you are looking to distribute a self-contained application that you can just run from the command line without unpacking.

To solve this problem, many developers use “shaded” jars. A shaded jar simply 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 you are actually using 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 allows you to actually nest jars directly.

E.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 BOOT-INF/classes directory. Dependencies should be placed in a nested BOOT-INF/lib directory.

E.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 WEB-INF/lib directory. Any dependencies that are required when running embedded but are not required when deploying to a traditional web container should be placed in WEB-INF/lib-provided.

E.2 Spring Boot’s “JarFile” class

The core class used to support loading nested jars is org.springframework.boot.loader.jar.JarFile. It allows you to 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:

myapp.jar
+-------------------+-------------------------+
| /BOOT-INF/classes | /BOOT-INF/lib/mylib.jar |
|+-----------------+||+-----------+----------+|
||     A.class      |||  B.class  |  C.class ||
|+-----------------+||+-----------+----------+|
+-------------------+-------------------------+
 ^                    ^           ^
 0063                 3452        3980

The example above shows how A.class can be found in /BOOT-INF/classes in myapp.jar position 0063. B.class from the nested jar can actually be found in myapp.jar position 3452 and C.class is at position 3980.

Armed with this information, we can load specific nested entries by simply seeking to the appropriate part of the outer jar. We don’t need to unpack the archive and we don’t need to read all entry data into memory.

E.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. The getURL() method will return a URL that opens a java.net.JarURLConnection compatible connection and can be used with Java’s URLClassLoader.

E.3 Launching executable jars

The org.springframework.boot.loader.Launcher class is a special bootstrap class that is used as an executable jars main entry point. It is the actual Main-Class in your jar file and it’s used to setup an appropriate URLClassLoader and ultimately call your main() method.

There are 3 launcher subclasses (JarLauncher, WarLauncher and PropertiesLauncher). Their purpose is to load resources (.class files etc.) from nested jar files or war files in directories (as opposed to explicitly on the classpath). In the case of JarLauncher and WarLauncher the nested paths are fixed. JarLauncher looks in BOOT-INF/lib/ and WarLauncher looks in WEB-INF/lib/ and WEB-INF/lib-provided/ so you just add extra jars in those locations if you want more. The PropertiesLauncher looks in BOOT-INF/lib/ in your application archive by default, but you can add additional locations by setting an environment variable LOADER_PATH or loader.path in application.properties (comma-separated list of directories or archives).

E.3.1 Launcher manifest

You need to specify an appropriate Launcher as the Main-Class attribute of META-INF/MANIFEST.MF. The actual class that you want to launch (i.e. the class that you wrote that contains a main method) should be specified in the Start-Class attribute.

For example, here is 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:

Main-Class: org.springframework.boot.loader.WarLauncher
Start-Class: com.mycompany.project.MyApplication

	Note
	You do not need to specify `Class-Path` entries in your manifest file, the classpath will be deduced from the nested jars.

E.3.2 Exploded archives

Certain PaaS implementations may choose to unpack archives before they run. For example, Cloud Foundry operates in this way. You can run an unpacked archive by simply starting the appropriate launcher:

$ unzip -q myapp.jar
$ java org.springframework.boot.loader.JarLauncher

E.4 PropertiesLauncher Features

PropertiesLauncher has a few special features that can be enabled with external properties (System properties, environment variables, manifest entries or application.properties).

Key	Purpose
`loader.path`	Comma-separated Classpath, e.g. `lib,${HOME}/app/lib`. Earlier entries take precedence, just like a regular `-classpath` on the `javac` command line.
`loader.home`	Location of additional properties file, e.g. `/opt/app` (defaults to `${user.dir}`)
`loader.args`	Default arguments for the main method (space separated)
`loader.main`	Name of main class to launch, e.g. `com.app.Application`.
`loader.config.name`	Name of properties file, e.g. `loader` (defaults to `application`).
`loader.config.location`	Path to properties file, e.g. `classpath:loader.properties` (defaults to `application.properties`).
`loader.system`	Boolean flag to indicate that all properties should be added to System properties (defaults to `false`)

When specified as environment variables or manifest entries, the following names should be used:

Key	Manifest entry	Environment variable
`loader.path`	`Loader-Path`	`LOADER_PATH`
`loader.home`		`LOADER_HOME`
`loader.args`	`Loader-Args`	`LOADER_ARGS`
`loader.main`	`Start-Class`	`LOADER_MAIN`
`loader.config.location`		`LOADER_CONFIG_LOCATION`
`loader.system`		`LOADER_SYSTEM`

	Tip
	Build plugins automatically move the `Main-Class` attribute to `Start-Class` when the fat jar is built. If you are using that, specify the name of the class to launch using the `Main-Class` attribute and leave out `Start-Class`.

loader.home is the directory location of an additional properties file (overriding the default) as long as loader.config.location is not specified.
loader.path can contain directories (scanned recursively for jar and zip files), archive paths, or wildcard patterns (for the default JVM behavior).
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 PropertiesLauncher behaves the same as JarLauncher when no additional configuration is provided.
Placeholder replacement is done from System and environment variables plus the properties file itself on all values before use.

E.5 Executable jar restrictions

There are a number of restrictions that you need to consider when working with a Spring Boot Loader packaged application.

E.5.1 Zip entry compression

The ZipEntry for a nested jar must be saved using the ZipEntry.STORED method. 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.

E.5.2 System ClassLoader

Launched applications should use Thread.getContextClassLoader() when loading classes (most libraries and frameworks will do this by default). Trying to load nested jar classes via ClassLoader.getSystemClassLoader() will fail. Please be aware that java.util.Logging always uses the system classloader, for this reason you should consider a different logging implementation.

E.6 Alternative single jar solutions

If the above restrictions mean that you cannot use Spring Boot Loader the following alternatives could be considered:

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