[appendix]
[[executable-jar]]
== The Executable Jar Format
The `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 section provides some background.



[[executable-jar-nested-jars]]
=== 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.



[[executable-jar-jar-file-structure]]
==== The Executable Jar File Structure
Spring Boot Loader-compatible jar files should be structured in the following way:

[indent=0]
----
	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.



[[executable-jar-war-file-structure]]
==== The Executable War File Structure
Spring Boot Loader-compatible war files should be structured in the following way:

[indent=0]
----
	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`.



[[executable-jar-jarfile]]
=== Spring Boot's "`JarFile`" Class
The core class used to support loading nested jars is
`org.springframework.boot.loader.jar.JarFile`. 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:

[indent=0]
----
	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 `/BOOT-INF/classes` in
`myapp.jar` at position `0063`. `B.class` from the nested jar can actually be found in
`myapp.jar` at position `3452`, and `C.class` is at position `3980`.

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.



[[executable-jar-jarfile-compatibility]]
==== 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 returns a `URL` that
opens a connection compatible with `java.net.JarURLConnection` and can be used with Java's
`URLClassLoader`.



[[executable-jar-launching]]
=== Launching Executable Jars
The `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
`main()` method.

There are three launcher subclasses (`JarLauncher`, `WarLauncher`, and
`PropertiesLauncher`). 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 `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/`. You can 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 called `LOADER_PATH`
or `loader.path` in `loader.properties` (which is a comma-separated list of directories,
archives, or directories within archives).



[[executable-jar-launcher-manifest]]
==== 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 (that is, the class that
contains a `main` method) should be specified  in the `Start-Class`
attribute.

The following example shows a typical `MANIFEST.MF` for an executable jar file:

[indent=0]
----
	Main-Class: org.springframework.boot.loader.JarLauncher
	Start-Class: com.mycompany.project.MyApplication
----

For a war file, it would be as follows:

[indent=0]
----
	Main-Class: org.springframework.boot.loader.WarLauncher
	Start-Class: com.mycompany.project.MyApplication
----

NOTE: You need not specify `Class-Path` entries in your manifest file. The classpath
is deduced from the nested jars.



[[executable-jar-exploded-archives]]
==== Exploded Archives
Certain PaaS implementations may choose to unpack archives before they run. For example,
Cloud Foundry operates this way. You can run an unpacked archive by starting
the appropriate launcher, as follows:

[indent=0]
----
	$ unzip -q myapp.jar
	$ java org.springframework.boot.loader.JarLauncher
----



[[executable-jar-property-launcher-features]]
=== `PropertiesLauncher` Features

`PropertiesLauncher` has a few special features that can be enabled with external
properties (System properties, environment variables, manifest entries, or
`loader.properties`). The following table describes these properties:

|===
|Key |Purpose

|`loader.path`
|Comma-separated Classpath, such as `lib,$\{HOME}/app/lib`. Earlier entries take
 precedence, like a regular `-classpath` on the `javac` command line.

|`loader.home`
|Used to resolve relative paths in `loader.path`. For example, given `loader.path=lib`,
 then `${loader.home}/lib` is a classpath location (along with all jar files in that
 directory). This property is also used to locate a `loader.properties` file, as in the
 following example `file:///opt/app`
 It defaults to `${user.dir}`.

|`loader.args`
|Default arguments for the main method (space separated).

|`loader.main`
|Name of main class to launch (for example, `com.app.Application`).

|`loader.config.name`
|Name of properties file (for example, `launcher`) It defaults to `loader`.

|`loader.config.location`
|Path to properties file (for example, `classpath:loader.properties`). It defaults to
 `loader.properties`.

|`loader.system`
|Boolean flag to indicate that all properties should be added to System properties
 It 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_HOME`

|`loader.args`
|`Loader-Args`
|`LOADER_ARGS`

|`loader.main`
|`Start-Class`
|`LOADER_MAIN`

|`loader.config.location`
|`Loader-Config-Location`
|`LOADER_CONFIG_LOCATION`

|`loader.system`
|`Loader-System`
|`LOADER_SYSTEM`

|===

TIP: Build plugins automatically move the `Main-Class` attribute to `Start-Class` when
the fat jar is built. If you use that, specify the name of the class to launch by using
the `Main-Class` attribute and leaving out `Start-Class`.

The following rules apply to working with `PropertiesLauncher`:

* `loader.properties` is 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.home` is the directory location of an additional properties file
  (overriding the default) only when `loader.config.location` is not specified.
* `loader.path` can 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.home` or anywhere in the file system with a
  `jar:file:` prefix.
* `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.
* `loader.path` can not be used to configure the location of `loader.properties` (the
  classpath used to search for the latter is the JVM classpath when `PropertiesLauncher`
  is launched).
* 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.



[[executable-jar-restrictions]]
=== Executable Jar Restrictions
You need to consider the following restrictions when working with a Spring
Boot Loader packaged application:



[[executable-jar-zip-entry-compression]]
* Zip entry compression:
The `ZipEntry` for a nested jar must be saved by 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.



[[executable-jar-system-classloader]]
* 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 `ClassLoader.getSystemClassLoader()` fails.
`java.util.Logging` always uses the system classloader. For this reason, you should
consider a different logging implementation.



[[executable-jar-alternatives]]
=== Alternative Single Jar Solutions
If the preceding restrictions mean that you cannot use Spring Boot Loader, consider the
following alternatives:

* https://maven.apache.org/plugins/maven-shade-plugin/[Maven Shade Plugin]
* http://www.jdotsoft.com/JarClassLoader.php[JarClassLoader]
* https://sourceforge.net/projects/one-jar/[OneJar]
* https://imperceptiblethoughts.com/shadow/[Gradle Shadow Plugin]

