浅析java类加载源码及测试浅析java类加载源码及测试

浅析java类加载源码及测试时间：20180225

类加载ClassLoader的源码中最重要的是ClassLoader方法、loadClass方法、findClass方法及defineClass方法，至于其他的方法不做解释。

// -- Class -- //通过name(二进制字节码)加载类，并按双亲委派的算法去执行 //findLoadedClass在已经加载的类中去寻找 //任何加载器都有父加载器 //如果某个加载器的父加载器为空（NULL），则父加载器为启动加载器（Bootstrap ClassLoader,虚拟机默认的加载器） //如果要自己实现加载器，需要重载findClass()方法 //没有找到类时throwsClassNotFoundException异常protected Class loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (getClassLoadingLock(name)) { //1. 检查当前类是否已经被加载 Class c = findLoadedClass(name); //2.如果当前类没有被加载过 if (c == null) { long t0 = System.nanoTime(); try { //3.并且当前加载器的父加载器不为空，即不为启动加载器 if (parent != null) { //4.1 循环委托给自己的父加载器（Ext或者App） c = parent.loadClass(name, false); } else { //4.2 循环委托给自己的父加载器（Bootstrap ） //返回通过BootStrap类加载器加载得到的类 c = findBootstrapClassOrNull(name); } } catch (ClassNotFoundException e) { // ClassNotFoundException thrown if class not found // from the non-null parent class loader }if (c == null) { // If still not found, then invoke findClass in order // to find the class. long t1 = System.nanoTime(); c = findClass(name); // this is the defining class loader; record the stats sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0); sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1); sun.misc.PerfCounter.getFindClasses().increment(); } } if (resolve) { resolveClass(c); } return c; } }//返回通过BootStrap类加载器加载得到的类 private Class findBootstrapClassOrNull(String name) { if (!checkName(name)) return null; return findBootstrapClass(name); }

代码解释：按照包含关系（非继承关系）

自定义加载器--->父加载器（自定义启动器or应用加载器App ClassLoader）--->父加载器（Ext ClassLoader）--->父加载器（启动加载器Bootstrap ClassLoader）。
代码中利用递归不停查找类的加载器的父加载器。直到最终找到的父加载器为NULL也就是为启动加载器时。
Bootstrap ClassLoader加载器中调用findBootstrapClassOrNull()方法首先查找缓存，如果没有找到的话，就去找自己规定的路径下，也就是sun.mic.boot.class下面的路径，找到就返回，没有找到，就让子加载器自己去找（递归实现）。
ExtClassLoader查找不成功，App ClassLoader就自己去查找。找到就返回。如果没有找到就让子加载器去找。如果子加载器（自定义加载器没有找到）？就会抛出异常（hrows ClassNotFoundException异常）。
上面代码及解释中。说明了双亲委派的加载流程。我们可以发现委派是从下向上。然后具体查找过程却是自上而下。如图解中。

图解双亲委派模型

文章图片
image.png

/** * Finds the class with the specified binary name. * This method should be overridden by class loader implementations that * follow the delegation model for loading classes, and will be invoked by * the {@link #loadClass loadClass} method after checking the * parent class loader for the requested class.The default implementation * throws a ClassNotFoundException. * * @paramname *The binary name of the class * * @returnThe resulting Class object * * @throwsClassNotFoundException *If the class could not be found * * @since1.2 */ protected Class findClass(String name) throws ClassNotFoundException { throw new ClassNotFoundException(name); }

/** * Converts an array of bytes into an instance of class Class, * with an optional ProtectionDomain.If the domain is * null, then a default domain will be assigned to the class as * specified in the documentation for {@link #defineClass(String, byte[], * int, int)}.Before the class can be used it must be resolved. * * The first class defined in a package determines the exact set of * certificates that all subsequent classes defined in that package must * contain.The set of certificates for a class is obtained from the * {@link java.security.CodeSource CodeSource} within the * ProtectionDomain of the class.Any classes added to that * package must contain the same set of certificates or a * SecurityException will be thrown.Note that if * name is null, this check is not performed. * You should always pass in the binary name of the * class you are defining as well as the bytes.This ensures that the * class you are defining is indeed the class you think it is. * * The specified name cannot begin with "java.", since * all classes in the "java.* packages can only be defined by the * bootstrap class loader.If name is not null, it * must be equal to the binary name of the class * specified by the byte array "b", otherwise a {@link * NoClassDefFoundError NoClassDefFoundError} will be thrown. * * @paramname *The expected binary name of the class, or *null if not known * * @paramb *The bytes that make up the class data. The bytes in positions *off through off+len-1 should have the format *of a valid class file as defined by *The Java™ Virtual Machine Specification. * * @paramoff *The start offset in b of the class data * * @paramlen *The length of the class data * * @paramprotectionDomain *The ProtectionDomain of the class * * @returnThe Class object created from the data, *and optional ProtectionDomain. * * @throwsClassFormatError *If the data did not contain a valid class * * @throwsNoClassDefFoundError *If name is not equal to the binary *name of the class specified by b * * @throwsIndexOutOfBoundsException *If either off or len is negative, or if *off+len is greater than b.length. * * @throwsSecurityException *If an attempt is made to add this class to a package that *contains classes that were signed by a different set of *certificates than this class, or if name begins with *"java.". */ protected final Class defineClass(String name, byte[] b, int off, int len, ProtectionDomain protectionDomain) throws ClassFormatError { protectionDomain = preDefineClass(name, protectionDomain); String source = defineClassSourceLocation(protectionDomain); Class c = defineClass1(name, b, off, len, protectionDomain, source); postDefineClass(c, protectionDomain); return c; }

三、举例

例子1

package com.test.jvm; import java.io.ByteArrayOutputStream; import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.InputStream; public class MyClassLoader extends ClassLoader{ private String name; //加载器名称 private String path; //加载器名称//构造函数 public MyClassLoader(String name , String path) { super(); //让系统类加载器为该类父加载器 this.name = name; this.path = path; }//构造函数，制定父加载器 public MyClassLoader(ClassLoader parent, String name, String path) { super(parent); //显示的指定父加载器 this.name=name; this.path=path; } /** * 加载我们定义的类，通过我们定义的ClassLoader */ @Override protected Class findClass(String name)throws ClassNotFoundException{ byte[] data = https://www.it610.com/article/null; Class C = null; try { data = https://www.it610.com/article/readClassFileToByteArray(name); C= this.defineClass(name, data, 0, data.length); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } returnC; } //如果不重写 ToString 结果是 Demo :com.test.jvm.MyClassLoader@55f96302 //为name（全限定名/类名）对应类数据所在的地址，toString的作用是将地址转换为类名 //即加上下面三行代码后返回结果变为：Demo :TanShen @Override public String toString() { return this.name; } /** *获取硬盘.class文件中的字节码数据，将数据存储与数组中 *.class文件由.java文件经过cmd执行javc生成 *1.处理传入的name(com.test.jvm.Demo或者Demo) * @param name * @return * @throws Exception */ private byte[] readClassFileToByteArray(String name) throws Exception { name = name.replaceAll("\\.","/"); String filePath = this.path + name + ".class"; File file = new File(filePath); InputStream is = null; byte[] returnData = https://www.it610.com/article/null; ByteArrayOutputStream os = new ByteArrayOutputStream(); try { is = new FileInputStream(file); int tmp = 0; while((tmp = is.read()) != -1) { os.write(tmp); } returnData = os.toByteArray(); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); }finally { is.close(); os.close(); }return returnData; } }

package com.test.jvm; public class TestDemo { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { //new一个自定义的类加载器；并在N:/tmp/ 目录下放入Demo.java文件 MyClassLoader loader = new MyClassLoader("TanShen","N:/tmp/"); System.out.println("当前加载器： "+loader); System.out.println("loader父加载器： "+loader.getParent()); System.out.println("loader父父加载器： "+loader.getParent().getParent()); System.out.println("loader父父父加载器： "+loader.getParent().getParent().getParent()); Class c = loader.loadClass("Demo"); c.newInstance(); } }

Demo.java中代码（别忘记cmd中执行 javac Demo，进行编译生产Demo.class）

public class Demo{ public Demo(){ System.out.println("Demo :" + this.getClass().getClassLoader()); } }

以上执行结果：
当前加载器： TanShen
loader父加载器： sun.misc.Launcher$AppClassLoader@4e25154f
loader父父加载器： sun.misc.Launcher$ExtClassLoader@55f96302
loader父父父加载器： null
Demo :TanShen

文章图片
图解分析自定义类加载器进行类的加载过程

例子2

MyClassLoader.java无变化

在eclipse下新建一个Demo.java 如下：

package com.test.jvm; public class Demo { public Demo() { //调用本类的类加载器 System.out.println("A Demo : " + this.getClass().getClassLoader()); } }

package com.test.jvm; public class TestDemo { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { //new一个自定义的类加载器 MyClassLoader loader = new MyClassLoader("TanShen","N:/tmp/"); System.out.println("当前加载器： "+loader); System.out.println("loader父加载器： "+loader.getParent()); System.out.println("loader父父加载器： "+loader.getParent().getParent()); System.out.println("loader父父父加载器： "+loader.getParent().getParent().getParent()); Class c = loader.loadClass("com.test.jvm.Demo"); c.newInstance(); } }

本地Demo.java中代码，并将Demo.java放入N:\tmp\com\test\jvm路径下（别忘记cmd中执行 javac Demo）

package com.test.jvm; public class Demo{ public Demo(){ //调用本类的类加载器 System.out.println("Demo :" + this.getClass().getClassLoader()); } }

eclipse目录

文章图片
目录

eclipse执行结果：
当前加载器： TanShen
loader父加载器： sun.misc.Launcher$AppClassLoader@4e25154f
loader父父加载器： sun.misc.Launcher$ExtClassLoader@55f96302
loader父父父加载器： null
A Demo : sun.misc.Launcher$AppClassLoader@4e25154f

文章图片
图解分析自定义类加载器进行类的加载过程

例子3

其他不变仅仅TestDemo进行改变

package com.test.jvm; public class TestDemo { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { //new一个自定义的类加载器 MyClassLoader tanShenloader = new MyClassLoader("TanShen","N:/tmp/"); MyClassLoader wuKongloader = new MyClassLoader(tanShenloader,"WuKong","N:/tmp/"); System.out.println("当前加载器： "+wuKongloader); System.out.println("loader父加载器： "+wuKongloader.getParent()); System.out.println("loader父父加载器： "+wuKongloader.getParent().getParent()); System.out.println("loader父父父加载器： "+wuKongloader.getParent().getParent().getParent()); System.out.println("loader父父父加载器： "+wuKongloader.getParent().getParent().getParent().getParent()); Class c = wuKongloader.loadClass("com.test.jvm.Demo"); c.newInstance(); } }

结果：
当前加载器： WuKong
loader父加载器： TanShen
loader父父加载器： sun.misc.Launcher$AppClassLoader@4e25154f
loader父父父加载器： sun.misc.Launcher$ExtClassLoader@55f96302
loader父父父加载器： null
A Demo : sun.misc.Launcher$AppClassLoader@4e25154f

案例分析：
当前自定义的加载器为WuKong，并指定其父加载器为：TanShen（也为自定义加载器），根据双亲委派模型，会继续寻找TanShen父加载器AppClassLoader；继而ExtClassLoader，最总 null==BootStrapClassLoader；
最后是
A Demo : sun.misc.Launcher$AppClassLoader@4e25154f去执行加载类，为什么？因为向下查找类的过程，AppClassLoader在缓存中找到了类Demo（就是java工程目录下的文件）。

例子4

其他不变仅仅TestDemo进行改变

package com.test.jvm; public class TestDemo { public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { //new一个自定义的类加载器 MyClassLoader wuKongloader = new MyClassLoader(null,"WuKong","N:/tmp/"); System.out.println("当前加载器： "+wuKongloader); System.out.println("loader父加载器： "+wuKongloader.getParent()); Class c = wuKongloader.loadClass("com.test.jvm.Demo"); c.newInstance(); } }

结果
当前加载器： WuKong
loader父加载器： null
Demo :WuKong

图解分析

文章图片
指定自定义类加载器的父加载器为Null（BootstrapClassLoader）
【浅析java类加载源码及测试】参考
https://www.bilibili.com/video/av17748750/
http://blog.csdn.net/briblue/article/details/54973413
搜索“探索JVM底层奥秘ClassLoader源码分析与案例讲解“