类加载机制浅记

/A class loader is an object that is responsible for loading classes. The
* class <tt>ClassLoader</tt> is an abstract class.  Given the <a
* href="#name">binary name</a> of a class, a class loader should attempt to
* locate or generate data that constitutes a definition for the class.  A
* typical strategy is to transform the name into a file name and then read a
* "class file" of that name from a file system.
* 
* <p> Every {@link Class <tt>Class</tt>} object contains a {@link
* Class#getClassLoader() reference} to the <tt>ClassLoader</tt> that defined
*/ 

ClassLoader classLoader = ClassLoaderDemo.class.getClassLoader();  

protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded
        
        //注释给的比较清除，首先检查 类是否已经被加载了
        Class<?> c = findLoadedClass(name);
        //如果没有找到加载的类 返回为 null
        if (c == null) {
            //设置System.nanoTime()的返回值只和进程已运行的时间有关, 不受调系统时间影响. 后面计算时间有用的
            long t0 = System.nanoTime();
            try {
                //如果该类加载器有 父类 让父类尝试去加载 传入的name 类     也就是双亲委派的实现 让父类去先加载 
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                //如果没有 则让 接下来这个   实际是这个 本地方法 c++ 去调用加载  private native Class<?> findBootstrapClass(String name);
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                 // 如果文件不存在 自然父类 加载不到 抛出异常
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }
            //如果父类返回的同样为null 说明父加载没有成功 
            if (c == null) {
                
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                // findClass（）网上说此时父类无法加载则将调用自己的类加载方法  虽然其实现只是要抛出 文件没有找到的异常 但从 其注释上找到了答案
                c = findClass(name);
                
                //记录时间
                // 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;
    }
}

    /
     * Finds the class with the specified <a href="#name">binary name</a>.
     * This method should be overridden by class loader implementations that      应该实现 其委派模型去加载类
     * follow the delegation model for loading classes, and will be invoked  该方法应该在执行了父类检查类加载后被执行
     * the {@link #loadClass <tt>loadClass</tt>} method after checking the    它默认只实现了抛出异常 
     * parent class loader for the requested class.  The default implementation
     * throws a <tt>ClassNotFoundException</tt>.
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @since  1.2
     */





protected Class<?> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
}

/
 * Finds and loads the class with the specified name from the URL search   找到 并加载类  
 * path. Any URLs referring to JAR files are loaded and opened as needed  
 * until the class is found.
 *
 * @param name the name of the class   
 * @return the resulting class
 * @exception ClassNotFoundException if the class could not be found,
 *            or if the loader is closed.
 * @exception NullPointerException if {@code name} is {@code null}.
 */
protected Class<?> findClass(final String name)
    throws ClassNotFoundException
{
    final Class<?> result;
    try {
        result = AccessController.doPrivileged(
            new PrivilegedExceptionAction<Class<?>>() {
                public Class<?> run() throws ClassNotFoundException {
                    String path = name.replace('.', '/').concat(".class");
                    Resource res = ucp.getResource(path, false);
                    if (res != null) {
                        try {
                            return defineClass(name, res);
                        } catch (IOException e) {
                            throw new ClassNotFoundException(name, e);
                        }
                    } else {
                        return null;
                    }
                }
            }, acc);
    } catch (java.security.PrivilegedActionException pae) {
        throw (ClassNotFoundException) pae.getException();
    }
    if (result == null) {
        throw new ClassNotFoundException(name);
    }
    return result;
}

static class AppClassLoader extends URLClassLoader {

static class ExtClassLoader extends URLClassLoader {