分析AppClassLoader,ExtClassLoader 和URLClassLoader 的关系
测试代码:
class Hello { public String str = "Hello World"; public void fun() { System.out.println(str); } } public class Test { public static void main(String[] args) { Hello hello = new Hello(); hello.fun(); System.out.println("----------------------"); //Hello类的类加载器 ClassLoader classLoaderOfHello = Hello.class.getClassLoader(); System.out.println("Hello is Loaded by : "+classLoaderOfHello); System.out.println("----------------------"); //Hello类的类加载器的Class对象 Class AppClazz = classLoaderOfHello.getClass(); //分析Hello类的类加载器的Class对象的类继承关系 while(AppClazz != null) { System.out.println(AppClazz); AppClazz = AppClazz.getSuperclass(); } System.out.println("----------------------"); //取得扩展器加载器的类对象Class Class ExtClazz = classLoaderOfHello.getParent().getClass(); while(ExtClazz != null) { System.out.println(ExtClazz); ExtClazz = ExtClazz.getSuperclass(); } } }
结论:
1. 用户自定义的类是由 应用(系统)类加载器AppClassLoader加载
2. 在”父亲委托机制”中,扩展类加载器ExtClassLoader是AppClassLoader的父亲.
3. AppClassLoader 和 ExtClassLoader 都扩展于 URLClassLoader加载器.
4. 也同时说明AppClassLoader而非继承ExtClassLoader.
继承关系:
java.lang.Object
--- java.lang.ClassLoader
--- java.security.SecureClassLoader
--- java.net.URLClassLoader
--- sun.misc.Launcher$ExtClassLoader
java.lang.Object
--- java.lang.ClassLoader
--- java.security.SecureClassLoader
--- java.net.URLClassLoader
--- sun.misc.Launcher$AppClassLoader
其实很简单嘛,直接看AppClassLoader的源代码就可以了嘛,哈哈,终于找到了好东东,上
JDK7:http://download.java.net/openjdk/jdk7/
JDK6:http://download.java.net/openjdk/jdk6/
下载其源代码就可以了
现在直接来看其源代码:
/** * The class loader used for loading from java.class.path. * runs in a restricted security context. */ static class AppClassLoader extends URLClassLoader { static { ClassLoader.registerAsParallelCapable(); } public static ClassLoader getAppClassLoader(final ClassLoader extcl) throws IOException { final String s = System.getProperty("java.class.path"); final File[] path = (s == null) ? new File[0] : getClassPath(s); return AccessController.doPrivileged( new PrivilegedAction<AppClassLoader>() { public AppClassLoader run() { URL[] urls = (s == null) ? new URL[0] : pathToURLs(path); return new AppClassLoader(urls, extcl); } }); } /* * Creates a new AppClassLoader */ AppClassLoader(URL[] urls, ClassLoader parent) { super(urls, parent, factory); } /** * Override loadClass so we can checkPackageAccess. */ public Class loadClass(String name, boolean resolve) throws ClassNotFoundException { int i = name.lastIndexOf('.'); if (i != -1) { SecurityManager sm = System.getSecurityManager(); if (sm != null) { sm.checkPackageAccess(name.substring(0, i)); } } return (super.loadClass(name, resolve)); } /** * allow any classes loaded from classpath to exit the VM. */ protected PermissionCollection getPermissions(CodeSource codesource) { PermissionCollection perms = super.getPermissions(codesource); perms.add(new RuntimePermission("exitVM")); return perms; } /** * This class loader supports dynamic additions to the class path * at runtime. * * @see java.lang.instrument.Instrumentation#appendToSystemClassPathSearch */ private void appendToClassPathForInstrumentation(String path) { assert(Thread.holdsLock(this)); // addURL is a no-op if path already contains the URL super.addURL( getFileURL(new File(path)) ); } /** * create a context that can read any directories (recursively) * mentioned in the class path. In the case of a jar, it has to * be the directory containing the jar, not just the jar, as jar * files might refer to other jar files. */ private static AccessControlContext getContext(File[] cp) throws java.net.MalformedURLException { PathPermissions perms = new PathPermissions(cp); ProtectionDomain domain = new ProtectionDomain(new CodeSource(perms.getCodeBase(), (java.security.cert.Certificate[]) null), perms); AccessControlContext acc = new AccessControlContext(new ProtectionDomain[] { domain }); return acc; } }
哈,看了AppClassLoader的源代码后,大家明白了吧,AppClassLoader 继承了URLClassLoader,而且构造函数是直接调用URLClassLoader的构造
函数,loadClass(String name, boolean resolve)方法只是简单做了包的安全检查,然后就调用ClassLoader的loadClass(String name, boolean resolve)方法了,其它的话,也是差不多..所以其功能和URLClassLoader差不多...
在ExtClassLoader也差不多,大家看看源代码就明了的:
/*
* Creates a new ExtClassLoader for the specified directories.
*/
public ExtClassLoader(File[] dirs) throws IOException {
super(getExtURLs(dirs), null, factory);
}
private static File[] getExtDirs() {
String s = System.getProperty("java.ext.dirs");
File[] dirs;
if (s != null) {
StringTokenizer st =
new StringTokenizer(s, File.pathSeparator);
int count = st.countTokens();
dirs = new File[count];
for (int i = 0; i < count; i++) {
dirs[i] = new File(st.nextToken());
}
} else {
dirs = new File[0];
}
return dirs;
}
private static URL[] getExtURLs(File[] dirs) throws IOException {
Vector<URL> urls = new Vector<URL>();
for (int i = 0; i < dirs.length; i++) {
String[] files = dirs[i].list();
if (files != null) {
for (int j = 0; j < files.length; j++) {
if (!files[j].equals("meta-index")) {
File f = new File(dirs[i], files[j]);
urls.add(getFileURL(f));
}
}
}
}
URL[] ua = new URL[urls.size()];
urls.copyInto(ua);
return ua;
}
/*
* Searches the installed extension directories for the specified
* library name. For each extension directory, we first look for
* the native library in the subdirectory whose name is the value
* of the system property <code>os.arch</code>. Failing that, we
* look in the extension directory itself.
*/
public String findLibrary(String name) {
name = System.mapLibraryName(name);
URL[] urls = super.getURLs();
File prevDir = null;
for (int i = 0; i < urls.length; i++) {
// Get the ext directory from the URL
File dir = new File(urls[i].getPath()).getParentFile();
if (dir != null && !dir.equals(prevDir)) {
// Look in architecture-specific subdirectory first
// Read from the saved system properties to avoid deadlock
String arch = VM.getSavedProperty("os.arch");
if (arch != null) {
File file = new File(new File(dir, arch), name);
if (file.exists()) {
return file.getAbsolutePath();
}
}
// Then check the extension directory
File file = new File(dir, name);
if (file.exists()) {
return file.getAbsolutePath();
}
}
prevDir = dir;
}
return null;
}
private static AccessControlContext getContext(File[] dirs)
throws IOException
{
PathPermissions perms =
new PathPermissions(dirs);
ProtectionDomain domain = new ProtectionDomain(
new CodeSource(perms.getCodeBase(),
(java.security.cert.Certificate[]) null),
perms);
AccessControlContext acc =
new AccessControlContext(new ProtectionDomain[] { domain });
return acc;
}
}