Java类加载器总结

加载器自己的初始化顺序，按照BootstrapClassLoader->Extension ClassLoader ->AppClassLoader的顺序加载，其中

BootstrapClassLoader由虚拟机启动后自动初始化，由C++编写，BootstrapClassLoader启动后先后加载Extension ClassLoader(java对象)->AppClassLoader（Java对象）.

 

加载类对象时，顺序为AppClassLoader-》Extension ClassLoader->BootstrapClassLoader逐级向上查找是否已经加载，如果根加载器没有加载，则从根加载器开始一层层向下加载,即每层的加载器在各自默认的路径下进行加载，如 Bootstrap ClassLoader《-》sun.mic.boot.class下 , ExtClassLoader《-》java.ext.dirs下 ,最终没找到则抛出异常，这就是所谓的“双亲委托机制”，可以避免类对象的重复加载。

这里所谓的双亲委托机制，列出核心代码:

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // 首先，检测是否已经加载
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try { //这一段即为双亲委托机制
                    if (parent != null) {//父加载器不为空则调用父加载器的loadClass
                        c = parent.loadClass(name, false);
                    } else {             //父加载器为空则调用Bootstrap Classloader
                        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();
                    //父加载器没有找到，则调用findclass
                    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) {  //调用resolveClass()
                resolveClass(c);
            }
            return c;
        }
    }

  

 

参考文献：https://blog.youkuaiyun.com/briblue/article/details/54973413

                    https://www.cnblogs.com/doit8791/p/5820037.html