Spring Boot Thin Launcher

A "thin" jar launcher for java apps. Version 1.0.31.RELEASE is in Maven Central, snapshots are in https://repo.spring.io/libs-snapshot. See https://github.com/spring-projects/spring-boot/issues/1813 for more discussion and ideas.

Getting Started

The thin-launcher provides its own custom layout for the Spring Boot plugins. If this layout is used then the jar built by Spring Boot will be executable and thin.

NOTE: if you are using a snapshot version of the thin launcher you either need to build it locally or include the snapshot repository declarations. You can use https://start.spring.io to find suitable repository declarations for Maven and Gradle, or look at the samples in this project.

With Maven, build a Spring Boot application and add the layout. This means adding it to the Spring Boot plugin declaration:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <version>${spring-boot.version}</version>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot.experimental</groupId>
            <artifactId>spring-boot-thin-layout</artifactId>
            <version>1.0.31.RELEASE</version>
        </dependency>
    </dependencies>
</plugin>

and in Gradle, you need to add the thin-launcher plugin and (preferably) a maven-publish plugin with an explicit publication definition. You can use the newer id style declaration:

plugins {
    id 'org.springframework.boot' version '3.0.1'
    id 'io.spring.dependency-management' version '1.1.0'
    id 'java'
    id 'maven-publish'
    id 'org.springframework.boot.experimental.thin-launcher' version '1.0.31.RELEASE'
}

group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '17'

repositories {
    mavenLocal()
    mavenCentral()
    maven { url "https://repo.spring.io/snapshot" }
    maven { url "https://repo.spring.io/milestone" }
}

publishing {
    publications {
        maven(MavenPublication) {
            from components.java
        }
    }
}

or you can use the older apply style declaration:

buildscript {
    ext {
        springBootVersion = '2.7.6'
        wrapperVersion = '1.0.31.RELEASE'
    }
    repositories {
        mavenLocal()
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot.experimental:spring-boot-thin-gradle-plugin:${wrapperVersion}")
        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
    }
}
apply plugin: 'maven-publish'
apply plugin: 'org.springframework.boot.experimental.thin-launcher'

publishing {
    publications {
        maven(MavenPublication) {
            from components.java
        }
    }
}

The publication named "maven" is responsible for creating a pom.xml for your application.

For Spring Boot 2.x snapshot versions (or older releases) you also need a settings.gradle with the repository configuration for the plugin. e.g. (as generated by https://start.spring.io):

pluginManagement{
    repositories{
        maven{url'https://repo.spring.io/libs-snapshot'}
        gradlePluginPortal()
    }
    resolutionStrategy{
        eachPlugin{
            if (requested.id.id=='org.springframework.boot.experimental.thin-launcher'){
                useModule("org.springframework.boot.experimental:spring-boot-thin-gradle-plugin:${requested.version}")
            }
        }
    }
}

If you don't need snapshots you can leave it all out and rely on the defaults. If you are testing a change you made to the plugin you might want to put mavenLocal() in the repositories as well.

In Gradle you also need to generate a pom.xml or a thin.properties (unless you want to maintain it by hand). A pom.xml will be generated automatically by the "thinPom" task in the Thin Gradle plugin. It does this by calling out to the maven plugin and the dependency management plugin; the maven plugin is always present, and the dependency management plugin is present if you are using the Spring Boot plugin. To generate a pom.xml remember to apply the maven and Thin Gradle plugins.

NOTE: Gradle now has a maven-publish plugin that works with the newer "standard" configurations (e.g. runtimeOnly replaces runtime). It also works with the thin launcher plugin.

The generated pom goes in the normal maven place by default under META-INF/maven. You can configure the output directory by setting the "output" property of the "thinPom" task.

You can customize the generated pom.xml, or switch it off, by creating your own task in build.gradle and forcing the jar task to depend on it instead of "thinPom", or by simply not using the Thin Gradle plugin. Example (which just duplicates the default):

task createPom {
    doLast {
        pom {
            withXml(dependencyManagement.pomConfigurer)
        }.writeTo("build/resources/main/META-INF/maven/${project.group}/${project.name}/pom.xml")
    }
}

jar.dependsOn = [createPom]

Instead of or as well as a pom.xml you could generate a thin.properties using gradle thinProperties (the task is always registered by the Thin Gradle plugin but is not executed by default). By default it shows up in META-INF in the built resources, so you need to run it before the jar is built, either manually, or via a task dependency, e.g.

jar.dependsOn = [thinProperties]

The generated properties file is "computed" (it contains all the transitive dependencies), so if you have that, the dependencies from the pom.xml will be ignored.

If you look at the jar file produced by the build you will see that it is "thin" (a few KB), but executable with java -jar ....

How does it Work?

Inspect the app jar that you built (or one of the samples in this project) and notice that it is only a few KB. It is just a regular jar file with the app classes in it and one or two extra features. The things it needs to operate are:

The ThinJarWrapper class has been added.
Either a pom.xml and/or a META-INF/thin.properties which lists the dependencies of the app.

When the app runs the main method per the manifest is the ThinJarWrapper. Its job is to locate another jar file (the "launcher"). The wrapper downloads the launcher (if it needs to), or else uses the cached version in your local Maven repository.

The launcher then takes over and reads the pom.xml (if present) and the thin.properties, downloading the dependencies (and all transitives) as necessary, and setting up a new class loader with them all on the classpath. It then runs the application's own main method with that class loader. The pom.xml can be in the root of the jar or in the standard META-INF/maven location.

The app jar in the demo is built using the Spring Boot plugin and a custom Layout (so it only builds with Spring Boot 2.x and above).

Caching JARs

All jar files are cached in the local Maven repository, so if you are building and running the same app repeatedly, it should be faster after the first time, or if the local repo is already warm.

The local repository can be re-located by setting a System property "thin.root". For example to use the current directory:

$ java -Dthin.root=. -jar app/target/*.jar

This will download all the dependencies to ${thin.root}/repository, and look for Maven settings in ${thin.root}/settings.xml.

You can also do a "dry run", just to warm up the cache and not run the app, by setting a System property or command line argument "thin.dryrun" (to any value except "false"). In fact, since you don't need the application code for this (except the META-INF/thin.properties), you could run only the launcher, or the wrapper which is contained in the launcher for convenience. This is a useful trick for laying down a file system layer in a container image, for example.

NOTE: options for the ThinJarLauncher that are listed as -Dthin.* can also be provided as command line arguments (--thin.* per Spring Boot conventions), or as environment variables (THIN_* capitalized and underscored). The command line options are removed before passing down to the Boot app. The ThinJarWrapper also accepts system properties, environment variables or command line flags for its (smaller) set of optional arguments.

Build Tools

Maven

In addition to the Spring Boot layout there is an optional Maven plugin which can be used to do the dry run (download and cache the dependencies) for the current project, or for any project that has an executable thin jar in the same format. The "app" sample in this repo declares this plugin and inserts it into the "package" lifecycle:

<plugin>
    <groupId>org.springframework.boot.experimental</groupId>
    <artifactId>spring-boot-thin-maven-plugin</artifactId>
    <version>${wrapper.version}</version>
    <executions>
        <execution>
            <id>resolve</id>
            <goals>
                <goal>resolve</goal>
            </goals>
            <inherited>false</inherited>
        </execution>
    </executions>
</plugin>

After running the build, there is a deployable warm-cache and a copy of the executable jar at target/thin/root (by default):

$ cd samples/app
$ mvn package
$ cd target/thin/root
$ java -Dthin.root=. -jar app-0.0.1-SNAPSHOT.jar

The "simple" sample has the same feature, but it also downloads and warms up the cache for the "app" sample, so you could use the same build to run both apps if you felt like it.

The Maven plugin also has a properties mojo, so you can create or update thin.properties from the dependencies of the project directly. By default it creates a thin.properties in src/main/resources/META-INF, but you can change the output directory with the plugin configuration. Example:

$ cd samples/app
$ mvn spring-boot-thin:properties -Dthin.output=.

By default the thin.properties is "computed" (i.e. it contains all transitive dependencies), but you can switch to just the declared dependencies using the "compute" configuration flag in the plugin (or -Dthin.compute=false on the command line).

Gradle

The same features are available to Gradle users by adding the thin jar plugin (as described above).

The plugin creates 2 tasks for every jar task in the project, one that reolves the dependencies, and one that copies the jar into the same location to make it easy to launch. A "dry run" can be executed in Gradle by calling the "thinResolve" task defined by the plugin, e.g.

$ cd samples/simple
$ gradle thinResolve
$ cd build/thin/deploy
$ java -Dthin.root=. -jar simple-0.0.1-SNAPSHOT.jar

The default location for the cache is build/thin/root but this was changed in the build.gradle for that sample:

thinResolvePrepare {
    into new File("${buildDir}/thin/deploy")
}

NOTE: The "thinResolve" and "thinResolvePrepare" tasks are the default names for a single jar project. If your jar task is not called "jar", then the names are appended with the jar task name (capitalized), e.g. "thinResolveMyJar" for a task called "myJar"). If you have multiple jar tasks in the project, then each one has its own resolve tasks.

Deploying to Cloud Foundry (or Heroku)

The thin launcher (1.0.4 and above) adds an empty "lib" entry to the jar so that it matches the default detection algorithm for a Java application with the standard Java buildpack. As of version v4.12 of the Java buildpack the dependencies will be computed during staging (in the "compile" step of the buildpack), so you don't incur that cost on startup.

You can also save the staging cost, and resolve the dependencies locally before you push the app.

$ java -jar target/demo-0.0.1.jar --thin.dryrun --thin.root=target/thin/.m2
$ (cd target/thin; jar -xf ../demo-0.0.1,jar)
$ cf push myapp -p target/thin

(Note the use of a subdirectory .m2 to hold the local repository cache - this works because the root is the default HOME directory in a Cloud Foundry app.)

The Maven plugin has a "resolve" task with a flag unpack (or -Dthin.unpack on the command line) that creates the cache in the precise form that you need to push to Cloud Foundry. The unpack flag is false by default, so remember to set it if you want to use Maven to prepare the push.

Command Line Options

You can set a variety of options on the command line or with system properties (-D...). The thin.* properties are all removed from the command line before calling the main class, so the main class doesn't have to know how it was launched.

Option	Default	Description
`thin.main`	Start-Class in MANIFEST.MF	The main class to launch (for a Spring Boot app, usually the one with `@SpringBootApplication`)
`thin.dryrun`	false	Only resolve and download the dependencies. Don't run any main class. N.B. any value other than "false" (even empty) is true.
`thin.offline`	false	Switch to "offline" mode. All dependencies must be available locally (e.g. via a previous dry run) or there will be an exception.
`thin.force`	false	Force dependency resolution to happen, even if dependencies have been computed, and marked as "computed" in `thin.properties`.
`thin.classpath`	false	Only print the classpath. Don't run the main class. Two formats are supported: "path" and "properties". For backwards compatibility "true" or empty are equivalent to "path".
`thin.root`	`${user.home}/.m2`	The location of the local jar cache, laid out as a maven repository. The launcher creates a new directory here called "repository" if it doesn't exist.
`thin.libs`	`<empty>`	Additional classpath entries to append at runtime in the same form as you would use in `java -classpath ...`. If this property is defined then unresolved dependencies will be ignored when the classpath is computed, possibly leading to runtime class not found exceptions.
`thin.archive`	the same as the target archive	The archive to launch. Can be used to launch a JAR file that was build with a different version of the thin launcher, for instance, or a fat jar built by Spring Boot without the thin launcher.
`thin.parent`	`<empty>`	A parent archive to use for dependency management and common classpath entries. If you run two apps with the same parent, they will have a classpath that is the same, reading from left to right, until they actually differ.
`thin.location`	`file:.,classpath:/`	The path to directory containing thin properties files (as per `thin.name`), as a comma-separated list of resource locations (directories). These locations plus the same paths relative /META-INF will be searched.
`thin.name`	"thin"	The name of the properties file to search for dependency specifications and overrides.
`thin.profile`		Comma-separated list of profiles to use to locate thin properties. E.g. if `thin.profile=foo` the launcher searches for files called `thin.properties` and `thin-foo.properties`.
`thin.library`	`org.springframework.boot.experimental:spring-boot-thin-launcher:1.0.31.RELEASE`	A locator for the launcher library. Can be Maven coordinates (with optional `maven://` prefix), or a file (with optional `file://` prefix).
`thin.repo`	`https://repo.spring.io/snapshot` (also contains GA releases)	Base URL for the `thin.library` if it is in Maven form (the default).
`thin.launcher`	`org.springframework.boot.thin.ThinJarLauncher`	The main class in the `thin.library`. If not specified it is discovered from the manifest `Main-Class` attribute.
`thin.parent.first`	true	Flag to say that the class loader is "parent first" (i.e. the system class loader will be used as the default). This is the "standard" JDK class loader strategy. Setting it to false is similar to what is normally used in web containers and application servers.
`thin.parent.boot`	true	Flag to say that the parent class loader should be the boot class loader not the "system" class loader. The boot loader normally includes the JDK classes, but not the target archive, nor any agent jars added on the command line.
`thin.debug`	false	Flag to switch on some slightly verbose logging during the dependency resolution. Can also be switched on with `debug` (like in Spring Boot).
`thin.trace`	false	Super verbose logging of all activity during the dependency resolution and launch process. Can also be switched on with `trace`.

Any other thin.properties.* properties are used by the launcher to override or supplement the ones from thin.properties, so you can add additional individual dependencies on the command line using thin.properties.dependencies.* (for instance).

Downstream Tools

The default behaviour of the ThinJarWrapper is to locate and launch the ThinJarLauncher, but it can also run any main class you like by using the thin.library and thin.launcher properties. One of the main reasons to provide this feature is to be able to support "tools" that process the application jar (or whatever), for example to generate metadata, create file system layers, etc. To create a new tool, make an executable jar (it can even be thin) with a Main-Class in its manifest, and point to it with thin.library. The launched main class will find the same command line as the launched jar, but with --thin.library removed if it was there. It will also find a system property thin.source containing the location of the launched jar, or the original thin.archive if that was provided on the command line (this is the archive that contains the data to process normally). If the tool jar is thin, i.e. if the main class is ThinJarWrapper, then the thin.archive command line argument and system property will also be removed (to prevent an infinite loop, where the wrapper just runs itself over and over).

An example of a tool jar is the spring-boot-thin-tools-converter (see below). You could use that as a prototype if you wanted to create your own.

HOWTO Guides

How to Externalize the Properties File

Example command line showing to pick up an external properties file:

$ cat config/thin.properties
dependencies.spring-boot-starter-web: org.springframework.boot:spring-boot-starter-web
$ java -jar app.jar --thin.location=file:./config

How to Create a Docker File System Layer

Precompute the dependencies:

$ java -jar app.jar --thin.root=m2 --thin.dryrun
$ java -jar app.jar --thin.classpath=properties > thin.properties

Then build a docker image using a Dockerfile based on this:

FROM openjdk:8-jdk-alpine
VOLUME /tmp

ADD m2 m2
ADD app.jar app.jar
ADD thin.properties thin.properties

ENTRYPOINT [ "sh", "-c", "java -Djava.security.egd=file:/dev/./urandom -jar app.jar --thin.root=/m2" ]

EXPOSE 8080

The step to add a thin.properties is optional, as is its calculation (you could maintain a hand-written properties file inside the JAR as well).

How to Change Dependencies

You can change the runtime dependencies, by changing the thin.properties in the jar. You can also read a local thin.properties from the current working directory, or set a System property thin.name to change the local file name (defaults to thin). There is also a thin.profile (comma separated list) which is appended to thin.name, so additional libraries can be added using thin-{profile}.properties. Profile-specific properties are loaded last so they take precedence. Example to pick up an extra set of dependencies in thin-rabbit.properties:

$ java -jar myapp.jar --thin.profile=rabbit

Profile-specific thin.properties can be saved in the jar file (conventionally in META-INF), or in the current working directory by default.

NOTE: You can add or override thin.properties entries on the command line or with System properties using key names in thin.properties.* (the prefix thin.properties. is stripped).

How to Upgrade Spring Boot or Spring Cloud

If your main pom (or properties) file uses boms to manage dependency versions, you can change the version of the bom using thin.properties. E.g.

boms.spring-boot-dependencies=org.springframework.boot:spring-boot-dependencies:2.2.4.RELEASE
...

If your main pom uses properties to manage dependencies (e.g. via the Spring Boot starter parent), you can change the value of the property using thin.properties. E.g.

spring-boot.version=2.2.4.RELEASE
spring-cloud.version=Hoxton.SR1

where the pom has

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-dependencies</artifactId>
      <version>${spring-boot.version}</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-dependencies</artifactId>
      <version>${spring-cloud.version}</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>

How to Exclude a Transitive Dependency

You can exclude and remove dependencies by prepending a key in the properties file with exclusions.. E.g.

dependencies.spring-boot-starter-web=org.springframework.boot:spring-boot-starter-web
dependencies.spring-boot-starter-jetty=org.springframework.boot:spring-boot-starter-jetty
exclusions.spring-cloud-starter-tomcat=org.springframework.boot:spring-cloud-starter-tomcat

How to Convert a Thin Jar to a Fat Jar

There is a converter tool that you can use as a library in place of the launcher. It works by copying all of the libraries from a thin.root into the new jar. Example:

$ java -jar myapp.jar --thin.dryrun --thin.root=target/thin/root
$ java -jar myapp.jar --thin.library=org.springframework.boot.experimental:spring-boot-thin-tools-converter:1.0.31.RELEASE
$ java -jar myapp-exec.jar

Building

To build this project locally, use the maven wrapper in the top level

$ ./mvnw clean install

Then run the "app" jar:

$ java -jar ./app/target/*.jar

(It starts an empty Spring Boot app with Tomcat.)

You can also build the samples independently.

Classpath Computation

The launcher has some optional arguments that result in classpath computations, instead of running the Boot app. E.g.

$ java -jar myapp.jar --thin.classpath=path

prints out (on stdout) a class path in the form that can be used directly in java -cp. So this is a way to run the app from its main method (which is faster than using the launcher):

$ CLASSPATH=`java -jar myapp.jar --thin.classpath=path`
$ java -cp "$CLASSPATH:myapp.jar" demo.MyApplication

You can also compute the classpath using explicit name and profile parameters:

$ java -jar myapp.jar --thin.classpath=path --thin.name=app --thin.profile=dev

will look for app.properties and app-dev.properties to list the dependencies.

You can also specify a "parent" archive which is used to calculate a prefix for the classpath. Two apps that share a parent then have the same prefix, and can share classes using -Xshare:on. For example:

$ CP1=`java -jar myapp.jar --thin.classpath=path`
$ CP2=`java -jar otherapp.jar --thin.classpath=path --thin.parent=myapp.jar`

$ java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:off \
  -XX:DumpLoadedClassList=app.classlist \
  -noverify -cp $CP1:myapp.jar demo.MyApplication
$ java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:dump \
  -XX:SharedArchiveFile=app.jsa -XX:SharedClassListFile=app.classlist \
  -noverify -cp $CP1

$ java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:on \
  -XX:SharedArchiveFile=app.jsa -noverify -cp $CP1:myapp.jar demo.MyApplication
$ java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:on \
  -XX:SharedArchiveFile=app.jsa -noverify -cp $CP1:otherapp.jar demo.OtherApplication

the two apps at the end are sharing class data from app.jsa and will also start up faster (e.g. 6s startup goes down to 4s for a vanilla Eureka Server).

The thin launcher can be used to pre-compute its own dependency graph in the form of a properties file, which also speeds up the launch a bit, even if you still have to resolve all the jars (remotely or from the cache). To compute the dependency graph and output the result in the form of a properties file, just use the thin.classpath=properties flag on startup, e.g.

$ java -jar myapp.jar --thin.classpath=properties > thin.properties
$ java -jar myapp.jar

In this example the second startup will be slightly faster, depending on the size of the classpath, but up to a few hundred milliseconds on even a fast server, and more in a constrained environment.

It also works fine with profiles, so, for example, if myapp.jar contains a META-INF/thin-rapid.properties you could do this:

$ java -jar myapp.jar --thin.profile=rapid --thin.classpath=properties > thin-super.properties
$ java -jar myapp.jar --thin.profile=super

Note that the generated thin.properties in these examples contains the property value computed=true. This tells the dependency graph calculator that the dependencies provided do not need to have their transitive dependencies or versions computed. It is possible to combine more than one properties file if they have different values of the computed flag, but if they both also contain dependencies then only the computed ones will be used. Note that this means you can compute a profile using --thin.classpath=properties and use it as a cache, speeding up startup without affecting any other settings that might be in other thin.properties.

How to Change the Maven Local Repository

You can change the location of the local Maven repository, used to resolve and cache artifacts, using the standard Maven settings.xml file (with a top level element called <localRepository/>). You can also use a system property maven.repo.local (or maven.home which defaults to ${user.home}/.m2) when you launch the thin jar, but not a command line flag. The Maven and Gradle plugins respond to the settings.xml and also (with Maven) to -Dmaven.repo.local as a command line flag. When the launcher runs it also looks in ${thin.root}/.. for a settings.xml file and uses that in preference to any other location if it exists. (For historical reasons it also looks in ${thin.root}/.m2 but that directory is unlikely to exist.)

How to Configure a Proxy

The dependency resolution uses Maven libraries, and should respect the proxy settings in your settings.xml. The initial download of the launcher by the ThinJarWrapper uses regular JDK libraries so you need to specify the normal -D args for networking as well, unless you have the launcher already cached locally.

Using a Mirror for Maven Central

The thin launcher itself parses your local Maven settings.xml and uses the mirror settings there. To download the launcher itself, and bootstrap the process, you need to explicitly provide a thin.repo to the wrapper (the same as the mirror). You can do this on the command line when running the jar, using all the usual mechanisms. To run the build plugins resolve goals you can make the thin launcher jar a dependency of the plugin, to ensure it is cached locally before the plugin runs. E.g.

<pluginManagement>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot.experimental</groupId>
            <artifactId>spring-boot-thin-maven-plugin</artifactId>
            <version>${wrapper.version}</version>
            <dependencies>
                <dependency>
                    <groupId>org.springframework.boot.experimental</groupId>
                    <artifactId>spring-boot-thin-launcher</artifactId>
                    <classifier>exec</classifier>
                    <version>${wrapper.version}</version>
                </dependency>
            </dependencies>
        </plugin>
    </plugins>
</pluginManagement>

Or else you can set a project, system property or environment variable. E.g.

$ ./mvnw spring-boot-thin:resolve -Dthin.repo=http://localhost:8081/repository/maven-central

$ ./gradlew thinResolve -P thin.repo=http://localhost:8081/repository/maven-central

System properties (thin.repo) and environment variables (THIN_REPO) work too.

Using a Custom Repository for Dependencies

The thin launcher will use repository declarations from the pom.xml in the archive, and also from the ~/.m2/settings.xml (by default, but can be relocated using maven.home or thin.root). If you have a private repository that requires credentials to access it, the best choice is settings.xml. This also applies to the Gradle plugin because it executes the jar file, and the underlying mechanism is implemented using Maven.

NOTE: The thin.repo setting only applies to the location of the launcher jar itself. If your custom repo is not a mirror it is unlikely to be that location.

License

This project is Open Source software released under the Apache 2.0 license.

spring-projects-experimental / spring-boot-thin-launcher

readme