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Spring Boot - Loader

The Spring Boot Loader module allows JAR and WAR files that contain nested dependencies to be run using java -jar archive.jar.

Note: The quickest way to build a compatible archive is to use the spring-boot-maven-plugin or spring-boot-gradle-plugin.

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 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.

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>
 +-com
 |  +-mycompany
 |     + project
 |        +-YouClasses.class
 +-lib
    +-dependency1.jar
    +-dependency2.jar

Dependencies should be placed in a nested lib directory.

See executable-jar for an example project.

WAR file structure

Spring Boot Loader compatible war files should be structured in the following way:

example.jar
 |
 +-META-INF
 |  +-MANIFEST.MF
 +-org
 |  +-springframework
 |     +-boot
 |        +-loader
 |           +-<spring boot loader classes>
 +-WEB-INF
    +-classes
    |  +-com
    |     +-mycompany
    |        +-project
    |           +-YouClasses.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.

See executable-war for an example project.

RandomAccessJarFile

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

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

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

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

Compatibility

Spring Boot Loader strives to remain compatible with existing code and libraries. The RandomAccessJarFile extends from java.util.jar.JarFile and should work as a drop-in replacement. The RandomAccessJarFile.getURL() method will return a URL that opens a java.net.JarURLConnection compatible connection. RandomAccessJarFile URLs can be used with Java's URLClassLoader.

Launching

The org.springframework.boot.loader.Launcher class can be used to run your packaged application. It takes care of setting up an appropriate URLClassLoader and calling your main() method.

Launcher manifest

You need 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 a 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.

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

Restrictions

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

URLs

URLs for nested jar entries intentionally look and behave like standard jar URLs, You cannot, however, directly create a nested jar URL from a string:

URL url = classLoader.getResoure("/a/b.txt");
String s = url.toString(); // In the form 'jar:file:/file.jar!/nested.jar!/a/b.txt'
new URL(s); // This will fail

If you need to obtain URL using a String, ensure that you always provide a context URL to the constructor. This will ensure that the custom URLStreamHandler used to support nested jars is used.

URL url = classLoader.getResoure("/a");
new URL(url, "b.txt");

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. You can use the Spring Boot Maven or Gradle plugins to ensure that your archives are written correctly.

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.

Alternatives

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

Further Reading

For more information about any of the classes or interfaces discussed in the document please refer to the project Javadoc. If you need to build a compatible archives see the spring-boot-maven-plugin or spring-boot-gradle-plugin. If you are not using Maven or Gradle spring-boot-loader-tools provides some useful utilities to rewite existing zip files.