loadClass、findClass、defineClass

文章字数：265,阅读全文大约需要1分钟

loadClass

加载一个类，传入名字或位置

1. 调用findLoadedClass(String)检测是否这个类已经被加载
2. 没有被加载则调用父类的loadClass()方法，直到找到或者父类为空(说明是启动类加载器)
3. 到了启动类加载器都没有就调用findClass(String)

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 {
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                    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;
    }
}

findClass

1. 根据名称或位置加载.class字节码,然后使用defineClass
2. 通常由子类去实现
   loadClass的源码可以看到，最后如果还是找不到类就会调用findClass。但是ClassLoader的findClass是直接抛出异常，自定义类加载器需要实现此方法。

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

definclass

1. 把字节码转化为Class

---