在JVM中并行加载同一个库的不同版本

Question

我需要测试同一个库的不同版本之间的不同版本 - 以及运行时。因此我需要加载很多具有相同包名的类。 整个执行只从一个类中开始，其余所有类都依赖它。

我试图加载库＃1作为项目​​文件（即通过ClassPath类加载器）和库＃2作为jar并通过UrlClassLoader加载它的类。

问题是当我从UrlClassLoader加载一个类时 - 所有相关类都从库＃1中获取，它们已经由ClassPath Class加载器加载。

我知道类加载器从Bootstrap类加载器开始，然后结束您的自定义类加载器 - 但是您可以使Java加载的不仅仅是一个明确提到的类，而是它自定义的所有依赖树类加载器？

Answer 1

您可以从classpath加载一个版本，其余版本使用URLClassLoader，通过parent: null并使用反射来实例化成员。

另一种方法是使用自定义的类装载器的基础上的方法描述here可能看起来像：

import java.net.URL; 
import java.net.URLClassLoader; 
import java.net.URLStreamHandlerFactory; 

public class ParentLastClassLoader extends ClassLoader { 

    private ClassLoader parentClassLoader; 
    private URLClassLoader noParentClassLoader; 

    public ParentLastClassLoader(URL[] urls, ClassLoader parent) { 
     super(parent); 
     this.noParentClassLoader = new URLClassLoader(urls, null); 
    } 

    public ParentLastClassLoader(URL[] urls) { 
     super(Thread.currentThread().getContextClassLoader()); 
     this.noParentClassLoader = new URLClassLoader(urls, null); 
    } 

    public ParentLastClassLoader(URL[] urls, ClassLoader parent, URLStreamHandlerFactory factory) { 
     super(parent); 
     this.noParentClassLoader = new URLClassLoader(urls, null, factory); 
    } 

    @Override 
    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { 
     try { 
      // resolve using child class loader 
      return noParentClassLoader.loadClass(name); 
     } catch (ClassNotFoundException ex) { 
      // resolve using parent class loader 
      return super.loadClass(name, resolve); 
     } 
    } 
} 

我this Git repo创造了这样的POC。详细信息在README.md中。

POC内容

考虑到你有包括对以下几部分的多个版本（项目V1，V2，V3）库：

我们希望我们的图书馆有这样的接口：

public interface Core { 

    String getVersion(); 
    String getDependencyVersion(); 
} 

这是由实现：

package sample.multiversion; 

import sample.multiversion.deps.CoreDependency; 

public class ImportantCore implements Core { 

    private Utility utility; 
    private CoreDependency coreDependency; 

    public ImportantCore() { 
     utility = new Utility(); 
     coreDependency = new CoreDependency(); 
    } 

    public String getVersion() { 
     return utility.getVersion(); 
    } 

    public String getDependencyVersion() { 
     return coreDependency.getVersion(); 
    } 
} 

正在使用另一个类相同的库的：

package sample.multiversion; 

public class Utility { 

    public String getVersion() { 
     return "core-v1"; 
    } 
} 

最后库（项目V1，V2，V3）具有（项目v1dep，v2dep，v3dep）含有依赖性：

package sample.multiversion.deps; 

public class CoreDependency { 

    public String getVersion() { 
     return "core-dep-v1"; 
    } 
} 

然后我们可以加载所有三个版本：

1. V1 - 从文件
2. V2 - 从文件
3. V3 - 从classpath中

的代码将是：

// multiple versions of the same library to be used at the same time 
    URL v1 = Paths.get("./../v1/build/libs/v1.jar").toUri().toURL(); 
    URL v2 = Paths.get("./../v2/build/libs/v2.jar").toUri().toURL(); 

    // library dependencies 
    URL v1Dep = Paths.get("./../v1dep/build/libs/v1dep.jar").toUri().toURL(); 
    URL v2Dep = Paths.get("./../v2dep/build/libs/v2dep.jar").toUri().toURL(); 

    /** 
    * version 1 and 2 loaders 
    * - these loaders do not use the root loader - Thread.currentThread().getContextClassLoader() 
    * - using the root loader new URLClassLoader(new URL[]{v1, v1Dep}, Thread.currentThread().getContextClassLoader()); 
    * will solve any class with the root loader and if no class is found then the child loader will be used 
    * - because version 3 is loaded from classpath, the root loader should not be used to load version 1 and 2 
    * - null needs to be passed to parent argument, else will not work 
    */ 
    URLClassLoader loaderV1 = new URLClassLoader(new URL[]{v1, v1Dep}, null); 
    URLClassLoader loaderV2 = new URLClassLoader(new URL[]{v2, v2Dep}, null); 

    /** 
    * Use custom class loader for loading classes first from children and last from parent 
    */ 
    ParentLastClassLoader loaderV1Alt = new ParentLastClassLoader(new URL[]{v1, v1Dep}); 
    ParentLastClassLoader loaderV2Alt = new ParentLastClassLoader(new URL[]{v2, v2Dep}); 
    ParentLastClassLoader loaderV3Alt = new ParentLastClassLoader(new URL[]{}); 

    // get class from loader 
    Class<?> coreV1Class = loaderV1.loadClass("sample.multiversion.ImportantCore"); 
    Class<?> coreV2Class = loaderV2.loadClass("sample.multiversion.ImportantCore"); 

    // get class from loader - custom version 
    Class<?> coreV1AltClass = loaderV1Alt.loadClass("sample.multiversion.ImportantCore"); 
    Class<?> coreV2AltClass = loaderV2Alt.loadClass("sample.multiversion.ImportantCore"); 
    Class<?> coreV3AltClass = loaderV3Alt.loadClass("sample.multiversion.ImportantCore"); 

    // create class instance 
    Object coreV1Instance = coreV1Class.newInstance(); 
    Object coreV2Instance = coreV2Class.newInstance(); 

    // create class instance - obtained from custom class loader 
    Object coreV1AltInstance = coreV1AltClass.newInstance(); 
    Object coreV2AltInstance = coreV2AltClass.newInstance(); 

    // note that this is loaded from classpath 
    Core coreV3Instance = new ImportantCore(); 
    Core coreV3AltInstance = (Core)coreV3AltClass.newInstance(); 

    // get version 
    String v1Str = (String) coreV1Class.getMethod("getVersion").invoke(coreV1Instance); 
    String v2Str = (String) coreV2Class.getMethod("getVersion").invoke(coreV2Instance); 
    String v1AltStr = (String) coreV1AltClass.getMethod("getVersion").invoke(coreV1AltInstance); 
    String v2AltStr = (String) coreV2AltClass.getMethod("getVersion").invoke(coreV2AltInstance); 

    String v3Str = coreV3Instance.getVersion(); 
    String v3AltStr = coreV3AltInstance.getVersion(); 

    // get version of dependency 
    String v1DepStr = (String) coreV1Class.getMethod("getDependencyVersion").invoke(coreV1Instance); 
    String v2DepStr = (String) coreV2Class.getMethod("getDependencyVersion").invoke(coreV2Instance); 
    String v1AltDepStr = (String) coreV1AltClass.getMethod("getDependencyVersion").invoke(coreV1AltInstance); 
    String v2AltDepStr = (String) coreV2AltClass.getMethod("getDependencyVersion").invoke(coreV2AltInstance); 

    String v3DepStr = coreV3Instance.getDependencyVersion(); 
    String v3AltDepStr = coreV3AltInstance.getDependencyVersion(); 

    System.out.println(String.format("V1 loader :: version = '%s' :: dependency_version = '%s'", v1Str, v1DepStr)); 
    System.out.println(String.format("V2 loader :: version = '%s' :: dependency_version = '%s'", v2Str, v2DepStr)); 
    System.out.println(String.format("V3 loader :: version = '%s' :: dependency_version = '%s'", v3Str, v3DepStr)); 

    System.out.println(String.format("V1 custom loader :: version = '%s' :: dependency_version = '%s'", v1AltStr, v1AltDepStr)); 
    System.out.println(String.format("V2 custom loader :: version = '%s' :: dependency_version = '%s'", v2AltStr, v2AltDepStr)); 
    System.out.println(String.format("V3 custom loader :: version = '%s' :: dependency_version = '%s'", v3AltStr, v3AltDepStr)); 

注：Core接口可以是另一个项目的一部分由项目v1, v2, v3使用，这可以让我们投射新创建的实例并以类型安全的方式工作。但这可能不是每次都可行的。