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.
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.
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
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
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
B.class from the nested jar can actually be found in
C.class is at position
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.
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
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
There are 3 launcher subclasses (
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
WarLauncher the nested paths are fixed.
JarLauncher looks in
WarLauncher looks in
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.properties (comma-separated list of directories, archives, or directories within archives).
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
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
You do not need to specify
Class-Pathentries in your manifest file, the classpath will be deduced from the nested jars.
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
PropertiesLauncher has a few special features that can be enabled with external properties (System properties, environment variables, manifest entries or
PropertiesLaunchersupports loading properties from
loader.propertiesand also (for historic reasons)
application.properties. We recommend using
loader.propertiesexclusively, as support for
application.propertiesis deprecated and may be removed in the future.
|Comma-separated Classpath, e.g. |
|Used to resolve relative paths in |
|Default arguments for the main method (space separated)|
|Name of main class to launch, e.g. |
|Name of properties file, e.g. |
|Path to properties file, e.g. |
|Boolean flag to indicate that all properties should be added to System properties (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
Start-Classwhen the fat jar is built. If you are using that, specify the name of the class to launch using the
Main-Classattribute and leave out
loader.propertiesare searched for in
loader.homethen in the root of the classpath, then in
classpath:/BOOT-INF/classes. The first location that exists is used.
loader.homeis only the directory location of an additional properties file (overriding the default) as long as
loader.config.locationis not specified.
loader.pathcan contain directories (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.home, or 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 env vars, system properties,
loader.properties, exploded archive manifest, archive manifest.
There are a number of restrictions that you need to consider when working with a Spring Boot Loader packaged application.
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.
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.
If the above restrictions mean that you cannot use Spring Boot Loader the following alternatives could be considered: