JVM的基础——ClassLoader分类
首先普及ClassLoader的基础：所有的Java类都是由ClassLoader由class文件加载进内存的，对于一个类，其唯一标识就是类名+加载他的ClassLoader（亦即对于不同的 ClassLoader，即使是加载了同一个Class也不能互通，本质上是两个类），其基本的分类如下图：
BootstrapClassLoader是一个特殊的ClassLoader，负责启动时加载jre的类库。并不继承于ClassLoader，因为是jvm逻辑的一部分；
ExtClassLoader也会加载jre类库，但是会加载那些额外的扩展类库(jre\lib\ext目录)，到这个级别的
类加载器已经可以直接在代码中使用了； AppClassLoader是加载用户编写代码的
类加载器，会加载所有classpath路径下的类，所以这也是默认的系统 类加载器，默认通过调用ClassLoader.getSystemClassLoader()可以获取； 用户也可以自定义其它ClassLoader，继承于ClassLoader下的自定义
类加载器。
我们通过查看源码可以看到继承于ClassLoader的加载器(除去我们看不到的BootstrapClassLoader)在
ExtClassLoader
ExtClassLoader是
static class ExtClassLoader extendsURLClassLoader { private static volatile Launcher.ExtClassLoader instance; // Launcher通过这个方法获取ExtLoader实例，线程安全 public static Launcher.ExtClassLoader getExtClassLoader() throws IOException { if (instance == null) { Class var0 = Launcher.ExtClassLoader.class; synchronized( Launcher.ExtClassLoader.class) { if (instance == null) { instance = createExtClassLoader(); } } } return instance; } private static Launcher.ExtClassLoader createExtClassLoader() throws IOException { try { return ( Launcher.ExtClassLoader)AccessController .doPrivileged(new PrivilegedExceptionAction< Launcher.ExtClassLoader>() { public Launcher.ExtClassLoader run() throws IOException { File[] var1 = Launcher.ExtClassLoader.getExtDirs(); int var2 = var1.length; for(int var3 = 0; var3 < var2; ++var3) { MetaIndex.registerDirectory(var1[var3]); } return new Launcher.ExtClassLoader(var1); } }); } catch (PrivilegedActionException var1) { throw (IOException)var1.getException(); } } public ExtClassLoader(File[] var1) throws IOException { super(getExt URLs(var1), (ClassLoader)null, Launcher.factory); SharedSecrets.getJavaNetAccess().get URLClassPath(this).initLookupCache(this); } //获取ClassLoader的文件目录集合 //var0最后的值为 xxx\jre\lib\ext; 等 private static File[] getExtDirs() { String var0 = System.getProperty("java.ext.dirs"); File[] var1; if (var0 != null) { StringTokenizer var2 = new StringTokenizer(var0, File.pathSeparator); int var3 = var2.countTokens(); var1 = new File[var3]; for(int var4 = 0; var4 < var3; ++var4) { var1[var4] = new File(var2.nextToken()); } } else { var1 = new File[0]; } return var1; } //…………………… }
AppClassLoader
同ExtClassLoader也会内部静态类：
static class AppClassLoader extendsURLClassLoader { final URLClassPath ucp = SharedSecrets.getJavaNetAccess().get URLClassPath(this); public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException { final String var1 = System.getProperty("java.class.path"); final File[] var2 = var1 == null ? new File[0] : Launcher.getClassPath(var1); return (ClassLoader)AccessController.doPrivileged(new PrivilegedAction< Launcher.AppClassLoader>() { public Launcher.AppClassLoader run() { URL[] var1x = var1 == null ? new URL[0] : Launcher.pathTo URLs(var2); return new Launcher.AppClassLoader(var1x, var0); } }); } AppClassLoader( URL[] var1, ClassLoader var2) { //将var2作为parent super(var1, var2, Launcher.factory); this.ucp.initLookupCache(this); } public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException { int var3 = var1.lastIndexOf(46); if (var3 != -1) { SecurityManager var4 = System.getSecurityManager(); if (var4 != null) { var4.checkPackageAccess(var1.substring(0, var3)); } } if (this.ucp.knownToNotExist(var1)) { Class var5 = this.findLoadedClass(var1); if (var5 != null) { if (var2) { this.resolveClass(var5); } return var5; } else { throw new ClassNotFoundException(var1); } } else { return super.loadClass(var1, var2); } } }
双亲委派加载模型及其实现
在一个
创建ClassLoader：
publicLauncher() { Launcher.ExtClassLoader var1; try { //创建ExtClassLoader var1 = Launcher.ExtClassLoader.getExtClassLoader(); } catch (IOException var10) { throw new InternalError("Could not create extension class loader", var10); } try { //创建了AppClassLoader，将ExtClassLoader作为参数传入 //上面的super(var1, var2, Launcher.factory); 一行 this.loader = Launcher.AppClassLoader.getAppClassLoader(var1); } catch (IOException var9) { throw new InternalError("Could not create application class loader", var9); }
其中创建ClassLoader的super语句首行直接将传入的ClassLoader作为了parent：
private ClassLoader(Void unused, ClassLoader parent) {
this.parent = parent;
//…………
}
在loadClass方法中：
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
//有父加载器先从父加载器中加载class
if (parent != null) {
c = parent.loadClass(name, false);
//没有父加载器，则从bootStrap加载器中加载class
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
//都没有加载到，才从本加载器中进行class的加载
// 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;
}
}