loadClass（）、findClass（）、defineClass（）区别

loadClass（）

findLoadedClass(String)调用这个方法

protected Class loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // 首先，查看这个Class是否已经被加载
        Class c = findLoadedClass(name);
        //如果没有被加载，就查看父类加载器
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {
                    //递归调用loadClass()
                    c = parent.loadClass(name, false);
                } else {
                    //父类加载器是null，说明是启动类加载器，查找对应的Class
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            //如果都没有找到
            if (c == null) {
                // If still not found, then invoke findClass in order
                // to find the class.
                //调用findClass()
                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;
    }
}

findClass()

• 根据名称或位置加载.class字节码,然后使用defineClass
• 通常由子类去实现

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

class NetworkClassLoader extends ClassLoader {
        String host;
        int port;

         public Class findClass(String name) {
            byte[] b = loadClassData(name);
             return defineClass(name, b, 0, b.length);
         }

         private byte[] loadClassData(String name) {
             // load the class data from the connection
         }
}

definclass()

把字节码转化为Class

protected final Class defineClass(String name, byte[] b, int off, int len)
    throws ClassFormatError
{
    return defineClass(name, b, off, len, null);
}