# 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`. There are 3 launcher classes (`JarLauncher`, `WarLauncher` and `PropertiesLauncher`). Their purpose is to load resources (.class files etc.) from nested JAR files or JAR files in directories (as opposed to explicitly on the classpath). In the case of the `[Jar|War]Launcher` the nested paths are fixed `(lib/*.jar` and `lib-provided/*.jar` for the WAR case) so you just add extra JARs in those locations if you want more. The `PropertiesLauncher` looks in `lib/` by default, but you can add additional locations by setting an environment variable `LOADER_PATH`or `loader.path` in `application.properties` (colon-separated list of directories). > **Note:** The quickest way to build a compatible archive is to use the > [spring-boot-maven-plugin](../spring-boot-maven-plugin) or > [spring-boot-gradle-plugin](../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 | +- +-com | +-mycompany | + project | +-YouClasses.class +-lib +-dependency1.jar +-dependency2.jar ``` Dependencies should be placed in a nested `lib` directory. See [executable-jar](src/it/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 | +- +-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](src/it/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](../spring-boot-maven-plugin) or [Gradle](../spring-boot-gradle-plugin) 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: * [Maven Shade Plugin](http://maven.apache.org/plugins/maven-shade-plugin/) * [JarClassLoader](http://www.jdotsoft.com/JarClassLoader.php) * [OneJar](http://one-jar.sourceforge.net) ## 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](../spring-boot-maven-plugin) or [spring-boot-gradle-plugin](../spring-boot-gradle-plugin). If you are not using Maven or Gradle [spring-boot-loader-tools](../spring-boot-loader-tools) provides some useful utilities to rewite existing zip files.