如何在运行时动态加载Jars？

水深无声 2023-03-04 08:08 9阅读 0赞

本文翻译自：[How should I load Jars dynamically at runtime?][How should I load Jars dynamically at runtime]

*Why is it so hard to do this in Java?* **为什么用Java这么难？** *If you want to have any kind of module system you need to be able to load jars dynamically.* **如果要使用任何类型的模块系统，则需要能够动态加载jar。** *I'm told there's a way of doing it by writing your own `ClassLoader` , but that's a lot of work for something that should (in my mind at least) be as easy as calling a method with a jar file as its argument.* **有人告诉我，有一种方法可以通过编写自己的`ClassLoader` ，但是对于（至少在我看来）应该像调用带有jar文件作为其参数的方法一样容易的事情来说，这是很多工作。**

*Any suggestions for simple code that does this?* **对执行此操作的简单代码有什么建议吗？**

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### \#1楼 ###

参考：[https://stackoom.com/question/fO4/如何在运行时动态加载Jars][https_stackoom.com_question_fO4_Jars]

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### \#2楼 ###

*How about the [JCL class loader framework][] ?* **[JCL类加载器框架][JCL class loader framework]如何？** *I have to admit, I haven't used it, but it looks promising.* **我不得不承认，我没有使用过，但是看起来很有希望。**

*Usage example:* **用法示例：**

JarClassLoader jcl = new JarClassLoader();
    jcl.add("myjar.jar"); // Load jar file  
    jcl.add(new URL("http://myserver.com/myjar.jar")); // Load jar from a URL
    jcl.add(new FileInputStream("myotherjar.jar")); // Load jar file from stream
    jcl.add("myclassfolder/"); // Load class folder  
    jcl.add("myjarlib/"); // Recursively load all jar files in the folder/sub-folder(s)
    
    JclObjectFactory factory = JclObjectFactory.getInstance();
    // Create object of loaded class  
    Object obj = factory.create(jcl, "mypackage.MyClass");

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### \#3楼 ###

*If you are working on Android, the following code works:* **如果您使用的是Android，则以下代码适用：**

String jarFile = "path/to/jarfile.jar";
    DexClassLoader classLoader = new DexClassLoader(jarFile, "/data/data/" + context.getPackageName() + "/", null, getClass().getClassLoader());
    Class<?> myClass = classLoader.loadClass("MyClass");

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### \#4楼 ###

*Here is a version that is not deprecated.* **这是不推荐使用的版本。** *I modified the original to remove the deprecated functionality.* **我修改了原始文件以删除不推荐使用的功能。**

/**************************************************************************************************
     * Copyright (c) 2004, Federal University of So Carlos                                           *
     *                                                                                                *
     * All rights reserved.                                                                           *
     *                                                                                                *
     * Redistribution and use in source and binary forms, with or without modification, are permitted *
     * provided that the following conditions are met:                                                *
     *                                                                                                *
     *     * Redistributions of source code must retain the above copyright notice, this list of      *
     *       conditions and the following disclaimer.                                                 *
     *     * Redistributions in binary form must reproduce the above copyright notice, this list of   *
     *     * conditions and the following disclaimer in the documentation and/or other materials      *
     *     * provided with the distribution.                                                          *
     *     * Neither the name of the Federal University of So Carlos nor the names of its            *
     *     * contributors may be used to endorse or promote products derived from this software       *
     *     * without specific prior written permission.                                               *
     *                                                                                                *
     * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS                            *
     * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT                              *
     * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR                          *
     * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR                  *
     * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,                          *
     * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,                            *
     * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR                             *
     * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF                         *
     * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING                           *
     * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS                             *
     * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.                                   *
     **************************************************************************************************/
    /*
     * Created on Oct 6, 2004
     */
    package tools;
    
    import java.io.File;
    import java.io.IOException;
    import java.lang.reflect.Constructor;
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    import java.net.URL;
    import java.net.URLClassLoader;
    
    /**
     * Useful class for dynamically changing the classpath, adding classes during runtime. 
     */
    public class ClasspathHacker {
        /**
         * Parameters of the method to add an URL to the System classes. 
         */
        private static final Class<?>[] parameters = new Class[]{URL.class};
    
        /**
         * Adds a file to the classpath.
         * @param s a String pointing to the file
         * @throws IOException
         */
        public static void addFile(String s) throws IOException {
            File f = new File(s);
            addFile(f);
        }
    
        /**
         * Adds a file to the classpath
         * @param f the file to be added
         * @throws IOException
         */
        public static void addFile(File f) throws IOException {
            addURL(f.toURI().toURL());
        }
    
        /**
         * Adds the content pointed by the URL to the classpath.
         * @param u the URL pointing to the content to be added
         * @throws IOException
         */
        public static void addURL(URL u) throws IOException {
            URLClassLoader sysloader = (URLClassLoader)ClassLoader.getSystemClassLoader();
            Class<?> sysclass = URLClassLoader.class;
            try {
                Method method = sysclass.getDeclaredMethod("addURL",parameters);
                method.setAccessible(true);
                method.invoke(sysloader,new Object[]{ u }); 
            } catch (Throwable t) {
                t.printStackTrace();
                throw new IOException("Error, could not add URL to system classloader");
            }        
        }
    
        public static void main(String args[]) throws IOException, SecurityException, ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException, NoSuchMethodException{
            addFile("C:\\dynamicloading.jar");
            Constructor<?> cs = ClassLoader.getSystemClassLoader().loadClass("test.DymamicLoadingTest").getConstructor(String.class);
            DymamicLoadingTest instance = (DymamicLoadingTest)cs.newInstance();
            instance.test();
        }
    }

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### \#5楼 ###

*The solution proposed by jodonnell is good but should be a little bit enhanced.* **jodonnell提出的解决方案很好，但是应该有所增强。** *I used this post to develop my application with success.* **我使用这篇文章成功地开发了我的应用程序。**

## *Assign the current thread* **分配当前线程** ##

*Firstly we have to add* **首先，我们必须添加**

Thread.currentThread().setContextClassLoader(classLoader);

*or you will not able to load resource (such as spring/context.xml) stored into the jar.* **否则您将无法将存储的资源（例如spring / context.xml）加载到jar中。**

## *Do not include* **不包括** ##

*your jars into the parent class loader or you will not able to understand who is loading what.* **您的jar放入父类加载器，否则您将无法理解谁正在加载什么。**

*see also [Problem reloading a jar using URLClassLoader][]* **另请参见[使用URLClassLoader重新加载jar时出现问题][Problem reloading a jar using URLClassLoader]**

*However, OSGi framework remain the best way.* **但是，OSGi框架仍然是最好的方法。**

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### \#6楼 ###

*With **Java 9** , the answers with `URLClassLoader` now give an error like:* **使用Java 9时 ， `URLClassLoader`的答案现在会给出如下错误：**

java.lang.ClassCastException: java.base/jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to java.base/java.net.URLClassLoader

*This is because the class loaders used have changed.* **这是因为使用的类加载器已更改。** *Instead, to add to the system class loader, you can use the [Instrumentation][] API through an agent.* **相反，要添加到系统类加载器，可以通过代理使用[Instrumentation][] API。**

***Create an agent class:*** **创建一个代理类：**

package ClassPathAgent;
    
    import java.io.IOException;
    import java.lang.instrument.Instrumentation;
    import java.util.jar.JarFile;
    
    public class ClassPathAgent {
        public static void agentmain(String args, Instrumentation instrumentation) throws IOException {
            instrumentation.appendToSystemClassLoaderSearch(new JarFile(args));
        }
    }

***Add META-INF/MANIFEST.MF and put it in a JAR file with the agent class:*** **添加META-INF / MANIFEST.MF并将其放在具有代理类的JAR文件中：**

Manifest-Version: 1.0
    Agent-Class: ClassPathAgent.ClassPathAgent

***Run the agent:*** **运行代理：**

*This uses the [byte-buddy-agent][] library to add the agent to the running JVM:* **这使用[byte-buddy-agent][]库将代理添加到正在运行的JVM：**

import java.io.File;
    
    import net.bytebuddy.agent.ByteBuddyAgent;
    
    public class ClassPathUtil {
        private static File AGENT_JAR = new File("/path/to/agent.jar");
    
        public static void addJarToClassPath(File jarFile) {
            ByteBuddyAgent.attach(AGENT_JAR, String.valueOf(ProcessHandle.current().pid()), jarFile.getPath());
        }
    }

[How should I load Jars dynamically at runtime]: https://oldbug.net/q/fO4/How-should-I-load-Jars-dynamically-at-runtime
[https_stackoom.com_question_fO4_Jars]: https://stackoom.com/question/fO4/%E5%A6%82%E4%BD%95%E5%9C%A8%E8%BF%90%E8%A1%8C%E6%97%B6%E5%8A%A8%E6%80%81%E5%8A%A0%E8%BD%BDJars
[JCL class loader framework]: https://github.com/kamranzafar/JCL
[Problem reloading a jar using URLClassLoader]: https://stackoverflow.com/questions/3216780/problem-reloading-a-jar-using-urlclassloader/31624123#31624123
[Instrumentation]: https://docs.oracle.com/javase/9/docs/api/java/lang/instrument/Instrumentation.html
[byte-buddy-agent]: https://github.com/raphw/byte-buddy