用反射解析jar文件并执行里面Java代码
1、使用JarFile类读取jar包MANIFEST.MF中的Main-Class，如果没有设置Main-Class则调用方法中指定执行函数
2、解压jar文件
3、设置classPath
4、利用反射取得class和method，并运行该method
如下代码Run.java将被打包到jar文件中，使用Maven构建并打包：
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package com.chenzehe.runjar;public class Run {/** * @param args */public static void main(String[] args) {System.out.println("run......");if (args.length > 0) {System.out.println("args: " + args[0]);}}}?
开始打包没有指定在MANIFEST.MF中指定Main-Class，下面运行时只能指定要运行的类。
?RunJar.java为解析并执行该jar文件方法的类：
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package com.chenzehe.runjar;import java.io.File;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.OutputStream;import java.lang.reflect.Array;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.net.URL;import java.net.URLClassLoader;import java.util.ArrayList;import java.util.Arrays;import java.util.Enumeration;import java.util.jar.JarEntry;import java.util.jar.JarFile;import java.util.jar.Manifest;public class RunJar {/** * @param args */public static void main(String[] args) throws Throwable {String usage = "RunJar jarFile [mainClass] args...";if (args.length < 1) {System.err.println(usage);System.exit(-1);}int firstArg = 0;String fileName = args[firstArg++];File file = new File(fileName);// 从jar文件MANIFEST.MF中读取Main-ClassString mainClassName = null;JarFile jarFile;try {jarFile = new JarFile(fileName);} catch (IOException io) {throw new IOException("Error opening job jar: " + fileName).initCause(io);}Manifest manifest = jarFile.getManifest();if (manifest != null) {mainClassName = manifest.getMainAttributes().getValue("Main-Class");}jarFile.close();if (mainClassName == null) {if (args.length < 2) {System.err.println(usage);System.exit(-1);}// 如果MANIFEST.MF未设置Main-Class,则用户输入mainClassName = args[firstArg++];}mainClassName = mainClassName.replaceAll("/", ".");final File workDir = File.createTempFile("unjar", "",new File(System.getProperty("user.dir")));workDir.delete();workDir.mkdirs();if (!workDir.isDirectory()) {System.err.println("Mkdirs failed to create " + workDir);System.exit(-1);}// 解压jar文件unJar(file, workDir);// 设置classPathArrayList<URL> classPath = new ArrayList<URL>();classPath.add(new File(workDir + "/").toURL());classPath.add(file.toURL());ClassLoader loader = new URLClassLoader(classPath.toArray(new URL[0]));Thread.currentThread().setContextClassLoader(loader);// 利用反射取得class和methodClass<?> mainClass = Class.forName(mainClassName, true, loader);Method main = mainClass.getMethod("main", new Class[] { Array.newInstance(String.class, 0).getClass() });String[] newArgs = Arrays.asList(args).subList(firstArg, args.length).toArray(new String[0]);try {// 运行方法main.invoke(null, new Object[] { newArgs });} catch (InvocationTargetException e) {throw e.getTargetException();}} //解压jar包中的文件到toDir目录public static void unJar(File jarFile, File toDir) throws IOException {JarFile jar = new JarFile(jarFile);try {Enumeration entries = jar.entries();while (entries.hasMoreElements()) {JarEntry entry = (JarEntry) entries.nextElement();if (!entry.isDirectory()) {InputStream in = jar.getInputStream(entry);try {File file = new File(toDir, entry.getName());if (!file.getParentFile().mkdirs()) {if (!file.getParentFile().isDirectory()) {throw new IOException("Mkdirs failed to create "+ file.getParentFile().toString());}}OutputStream out = new FileOutputStream(file);try {byte[] buffer = new byte[8192];int i;while ((i = in.read(buffer)) != -1) {out.write(buffer, 0, i);}} finally {out.close();}} finally {in.close();}}}} finally {jar.close();}}}?
该代码为Hadoop中job提交到服务器调用的代码RunJar。
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