java9系列(八)Multi-Release JAR Files

序

本文主要研究下JEP 238: Multi-Release JAR Files

multi-release jar (MR JAR)

java9新支持了multi-release jar的功能，包括jar、javac、javap、jdeps等命令都能支持这个特性。所谓multi-release jar可以包含多个jdk版本的实现，在运行时JVM根据当前环境加载符合版本的class，这样可以使得jar包在兼容旧版本的同时尽可能早地尝试新版JDK的特性。

通过--release参数指定编译版本，依赖JEP 247: Compile for Older Platform Versions来编译成指定JDK版本的class

具体的变化就是META-INF目录下MANIFEST.MF文件新增了一个属性：

Multi-Release: true

然后META-INF目录下还新增了一个versions目录，如果是要支持java9，则在versions目录下有9的目录

实例

java8

• com.example.lang

public class StackHelper {

    public static String getCurrentStack() {
        System.out.println("java 8 stack");
        return Arrays.stream(Thread.currentThread()
                .getStackTrace())
                .map(element -> element.toString())
                .collect(Collectors.joining("\n"));
    }
}

• maven

<groupId>com.example</groupId>
    <artifactId>mr-jar-java</artifactId>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.target>1.8</maven.compiler.target>
        <maven.compiler.source>1.8</maven.compiler.source>
    </properties>
    <dependencies>
        <dependency>
            <groupId>com.example</groupId>
            <artifactId>mr-jar-java9</artifactId>
            <version>0.0.1-SNAPSHOT</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <configuration>
                    <archive>
                        <manifestEntries>
                            <Specification-Title>${project.artifactId}</Specification-Title>
                            <Implementation-Title>${project.artifactId}</Implementation-Title>
                            <Implementation-Version>${project.version}</Implementation-Version>
                            <Multi-Release>true</Multi-Release>
                        </manifestEntries>
                    </archive>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <executions>
                    <execution>
                        <id>add-java9-classes</id>
                        <phase>generate-resources</phase>
                        <goals>
                            <goal>unpack-dependencies</goal>
                        </goals>
                        <configuration>
                            <includeGroupIds>com.example</includeGroupIds>
                            <includeArtifactIds>mr-jar-java9</includeArtifactIds>
                            <excludeTransitives>true</excludeTransitives>
                            <outputDirectory>${project.build.directory}/generated-resources/META-INF/versions/9</outputDirectory>
                            <includes>**/*.class</includes>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>build-helper-maven-plugin</artifactId>
                <version>3.0.0</version>
                <executions>
                    <execution>
                        <id>add-resource</id>
                        <phase>generate-resources</phase>
                        <goals>
                            <goal>add-resource</goal>
                        </goals>
                        <configuration>
                            <resources>
                                <resource>
                                    <directory>${project.build.directory}/generated-resources/</directory>
                                </resource>
                            </resources>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

注意，这里依赖java9的jar包，然后Multi-Release设置为true，通过编译打包把java9的classes放到META-INF/versions/9目录下，原来java8的classes位置不变。

java9

• com.example.lang

public class StackHelper {

    public static String getCurrentStack() {
        System.out.println("java 9 stack");
        return StackWalker.getInstance()
                .walk(frames -> frames.map(Object::toString)
                        .collect(joining("\n")));
    }
}

• module-info.java

module mr.jar.java9 {
    exports com.example.lang;
}

• maven

<groupId>com.example</groupId>
    <artifactId>mr-jar-java9</artifactId>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.target>9</maven.compiler.target>
        <maven.compiler.source>9</maven.compiler.source>
    </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.6.2</version>
                <configuration>
                    <release>9</release>
                </configuration>
            </plugin>
        </plugins>
    </build>

打包

mvn clean install

• jar包内容如下

➜  mr-jar-java-0.0.1-SNAPSHOT git:(master) ✗ tree
.
├── META-INF
│   ├── MANIFEST.MF
│   ├── maven
│   │   └── com.example
│   │       └── mr-jar-java
│   │           ├── pom.properties
│   │           └── pom.xml
│   └── versions
│       └── 9
│           ├── com
│           │   └── example
│           │       └── lang
│           │           ├── StackHelper.class
│           │           └── StringHelper.class
│           └── module-info.class
└── com
    └── example
        └── lang
            ├── StackHelper.class
            └── StringHelper.class

12 directories, 8 files

• 查看MANIFEST.MF

Manifest-Version: 1.0
Created-By: Apache Maven 3.3.3
Built-By: demo
Build-Jdk: 9
Implementation-Title: mr-jar-java
Implementation-Version: 0.0.1-SNAPSHOT
Multi-Release: true
Specification-Title: mr-jar-java

• 确认java8 class版本

➜  mr-jar-java-0.0.1-SNAPSHOT git:(master) ✗ javap -verbose ./com/example/lang/StackHelper.class
Classfile /Users/demo/multi-release-jar-demo/mr-jar-java8/target/mr-jar-java-0.0.1-SNAPSHOT/com/example/lang/StackHelper.class
  Last modified 2018年3月7日; size 1901 bytes
  MD5 checksum 9fafe51ca3df481e8c2264753c281a9a
  Compiled from "StackHelper.java"
public class com.example.lang.StackHelper
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #15                         // com/example/lang/StackHelper
  super_class: #16                        // java/lang/Object
  interfaces: 0, fields: 0, methods: 3, attributes: 3

可以看到major版本是52

• 确认java9 class版本

➜  mr-jar-java-0.0.1-SNAPSHOT git:(master) ✗ javap -verbose ./META-INF/versions/9/com/example/lang/StackHelper.class
Classfile /Users/demo/multi-release-jar-demo/mr-jar-java8/target/mr-jar-java-0.0.1-SNAPSHOT/META-INF/versions/9/com/example/lang/StackHelper.class
  Last modified 2018年3月7日; size 1921 bytes
  MD5 checksum da6326681eb1b0584998a92178e22e27
  Compiled from "StackHelper.java"
public class com.example.lang.StackHelper
  minor version: 0
  major version: 53
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #14                         // com/example/lang/StackHelper
  super_class: #15                        // java/lang/Object
  interfaces: 0, fields: 0, methods: 3, attributes: 3

可以看到major版本是53

运行

• java8

java 8 stack
java.lang.Thread.getStackTrace(Thread.java:1559)
com.example.lang.StackHelper.getCurrentStack(StackHelper.java:14)
Java8Main.main(Java8Main.java:15)

• java9

java 9 stack
mr.jar.java9/com.example.lang.StackHelper.getCurrentStack(StackHelper.java:13)
mr.jar.java9.main/mr.jar.java9.main.Java9Main.main(Java9Main.java:17)

注意java9调用multi-release jar的工程，需要requires该module，然后编译运行都需要添加module-path

java --module-path ./target/classes:/Users/demo/.m2/repository/com/example/mr-jar-java/0.0.1-SNAPSHOT/mr-jar-java-0.0.1-SNAPSHOT.jar --module mr.jar.java9.main/mr.jar.java9.main.Java9Main

jar命令

上面的例子是利用maven插件来打包，也可以使用jar来打包multi-release jar，实例如下：

javac --release 8 -d out/8 mr-jar-java8/src/main/java/com/example/lang/StackHelper.java
javac --release 9 -d out/9 mr-jar-java9/src/main/java/com/example/lang/StackHelper.java
jar -cfm out/mr-jar-demo.jar ./MANIFEST.MF -C out/8 .
jar -uf out/mr-jar-demo.jar --release 9 -C out/9 .

这里的-c表示创建jar包，-C表示转去该目录执行不带-C的jar命令，-f指定jar包的文件名，-m指定manifest.mf文件，-u添加文件到jar包中
其中MANIFEST.MF包含Multi-Release: true

小结

java9提供的multi-release jar的功能，可以在一个jar包打入多个jdk版本，同时在java9及以上的版本支持multi-release。它的好处就是比如从java8到java9的迁移，如果java8依赖的jar本身就是multi-release的，那么升级到java9就比较方便，不用再改maven依赖。不好的地方就是有过度设计的味道，一个jar包含多个版本的class，显得有些冗余。

doc

• JDK 9 features
• JEP 238: Multi-Release JAR Files
• Building Multi-Release JARs with Maven
• Multi-Release JARs: Good or Bad Idea?
• Creating Multi-Release JAR Files in IntelliJ IDEA
• The (Practical) Truth about Multi-Release JARs
• Understanding Java 9 Multi-Release Jar File with Example
• Multi-Release JARs: Good or Bad Idea?
• Java 9 - Multi-Release Jar Example