Java如何隔离Jar包引入的间接依赖

基础组件/中间件开发中经常以Jar包的形式提供给业务开发人员用。如何避免组件依赖的Jar和业务依赖的Jar包造成冲突呢？Java中的Jar hell是一个伤心的话题。

一 重新打包依赖包，修改包结构名

有工具可以帮助修改包名，参考 jarjar：https://github.com/shevek/jarjar。比如：

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java -jar jarjar-1.3.jar process ./rule.txt asm-commons-5.2.jar rasp-asm-commons-5.2.jar
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java -jar jarjar-1.3.jar process ./rule.txt asm-commons-5.2.jar rasp-asm-commons-5.2.jar
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rule org.objectweb.asm.**  org.objectweb.myasm.@1
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rule org.objectweb.asm.**  org.objectweb.myasm.@1

需要注意的是所有相互引用或者依赖的类都要做替换；另外需要注意验证配置文件中类/包路径也做了相应的更新。比如MATE-INF/MAINFEST.MF，META-INF/services等配置。

MATE-INF/MAINFEST.MF****META-INF/services二 maven-shade-plugin

这是一个maven插件（参考http://maven.apache.org/plugins/maven-shade-plugin/），maven项目代码编译打包阶段会自动进行包路径和引用的替换。

maven pom.xml文件中这个插件的配置参考如下

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---            <plugin>                <groupId>org.apache.maven.plugins</groupId>                <artifactId>maven-shade-plugin</artifactId>                <version>2.4</version>                <executions>                    <execution>                        <phase>package</phase>                        <goals>                            <goal>shade</goal>                        </goals>                        <configuration>                            <artifactSet>                                <excludes>                                    <exclude>org.abego.treelayout:org.abego.treelayout.core</exclude>                                </excludes>                            </artifactSet>                            <transformers>                                <transformer                                        implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">                                    <manifestEntries>                                        <Project-Version>${project.version}</Project-Version>                                        <Build-Time>${maven.build.timestamp}</Build-Time>                                        <Git-Commit>${git-commit-id.commit.id.abbrev}</Git-Commit>                                        <Git-Branch>${git-commit-id.branch}</Git-Branch>                                        <Last-Commit-User-Name>${git-commit-id.commit.user.name}</Last-Commit-User-Name>                                        <Last-Commit-User-Email>${git-commit-id.commit.user.email}                                        </Last-Commit-User-Email>                                    </manifestEntries>                                </transformer>                            </transformers>                            <relocations>                                <relocation>                                    <pattern>org.apache.commons.lang3</pattern>                                    <shadedPattern>org.apache.my-commons.lang3</shadedPattern>                                </relocation>                                <relocation>                                    <pattern>org.apache.commons.io</pattern>                                    <shadedPattern>org.apache.my-commons.io</shadedPattern>                                </relocation>                            </relocations>                        </configuration>                    </execution>                </executions>            </plugin>
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---
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-shade-plugin</artifactId>
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<version>2.4</version>
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<executions>
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<execution>
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<phase>package</phase>
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<goals>
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<goal>shade</goal>
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</goals>
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<configuration>
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<artifactSet>
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<excludes>
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<exclude>org.abego.treelayout:org.abego.treelayout.core</exclude>
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</excludes>
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</artifactSet>
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<transformers>
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<transformer
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implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
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<manifestEntries>
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<Project-Version>${project.version}</Project-Version>
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<Build-Time>${maven.build.timestamp}</Build-Time>
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<Git-Commit>${git-commit-id.commit.id.abbrev}</Git-Commit>
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<Git-Branch>${git-commit-id.branch}</Git-Branch>
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<Last-Commit-User-Name>${git-commit-id.commit.user.name}</Last-Commit-User-Name>
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<Last-Commit-User-Email>${git-commit-id.commit.user.email}
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</Last-Commit-User-Email>
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</manifestEntries>
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</transformer>
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</transformers>
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<relocations>
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<relocation>
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<pattern>org.apache.commons.lang3</pattern>
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<shadedPattern>org.apache.my-commons.lang3</shadedPattern>
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</relocation>
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<relocation>
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<pattern>org.apache.commons.io</pattern>
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<shadedPattern>org.apache.my-commons.io</shadedPattern>
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</relocation>
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</relocations>
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</configuration>
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</execution>
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</executions>
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</plugin>

三 自定义ClassLoader, 打破父委托的类加载方式

比较彻底的方式是自定义ClassLoader，并打破默认的双亲委托类加载机制。也就是自定义ClassLoader优先加载我们Jar包中的类。一个简单参考实现如下。

阿里Arthas中的自定义ClassLoader实现，其他如Greys，JVM-Sandbox也是类似的机制）。

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package com.taobao.arthas.agent;import java.net.URL;import java.net.URLClassLoader;/** * @author beiwei30 on 09/12/2016. */public class ArthasClassloader extends URLClassLoader {    public ArthasClassloader(URL[] urls) {        super(urls, ClassLoader.getSystemClassLoader().getParent());    }    @Override    protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {        final Class<?> loadedClass = findLoadedClass(name);        if (loadedClass != null) {            return loadedClass;        }        // 优先从parent（SystemClassLoader）里加载系统类，避免抛出ClassNotFoundException        if (name != null && (name.startsWith("sun.") || name.startsWith("java."))) {            return super.loadClass(name, resolve);        }        try {            Class<?> aClass = findClass(name);            if (resolve) {                resolveClass(aClass);            }            return aClass;        } catch (Exception e) {            // ignore        }        return super.loadClass(name, resolve);    }}
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package com.taobao.arthas.agent;
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import java.net.URL;
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import java.net.URLClassLoader;
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/**
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* @author beiwei30 on 09/12/2016.
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*/
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public class ArthasClassloader extends URLClassLoader {
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public ArthasClassloader(URL[] urls) {
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super(urls, ClassLoader.getSystemClassLoader().getParent());
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}
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@Override
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protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
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final Class<?> loadedClass = findLoadedClass(name);
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if (loadedClass != null) {
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return loadedClass;
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}
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// 优先从parent（SystemClassLoader）里加载系统类，避免抛出ClassNotFoundException
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if (name != null && (name.startsWith("sun.") || name.startsWith("java."))) {
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return super.loadClass(name, resolve);
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}
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try {
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Class<?> aClass = findClass(name);
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if (resolve) {
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resolveClass(aClass);
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}
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return aClass;
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} catch (Exception e) {
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// ignore
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}
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return super.loadClass(name, resolve);
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}
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}