谈谈ClassLoader

java.lang

public abstract class ClassLoader

通过委派模型(delegation model)进行class的加载。每个类加载器有一个关联的父类加载器。当加载一个类时，classLoader会委派给它的parent class loader加载该类。虚拟机内建的classLoader叫做bootstrap class loader,它没有父类加载器。

数组class（array classes）并不是由类加载器创建的，而是由Java运行时（Java runtime）在需要的时候自动创建的。

ClassLoader首先locate指定class的bytecode,然后将该类的bytecode转换成一个运行时的Class。运行时系统分别提供了加载bootstrap classes,extension classes和user classes的class loader。CLASSPATH环境变量指示运行时系统去哪里加载bytecode.

当一个Class被请求时，类加载器首先检查这个Class是否之前已被加载，如果没有，这个类加载器请求他的父加载器加载这个class。当父加载器没有找到这个class时，该类加载器尝试自己加载这个class.

在多线程环境下，自定义类加载器可能存在循环委派模型的问题，从而导致死锁：

Class Hierarchy:
class A extends B
class C extends D

ClassLoader Delegation Hierarchy:

Custom Classloader CL1:
directly loads class A
delegates to custom ClassLoader CL2 for class B

Custom Classloader CL2:
directly loads class C
delegates to custom ClassLoader CL1 for class D

Thread 1:
Use CL1 to load class A (locks CL1)
defineClass A triggers
loadClass B (try to lock CL2)

Thread 2:
Use CL2 to load class C (locks CL2)
defineClass C triggers
loadClass D (try to lock CL1)

加载一个Class会同步整个ClassLoader对象，可能导致死锁

JDK1.7引入parallel capable class loader，锁的粒度从classLoader变成了(classLoader+className),缩小了锁的粒度，解决了这个问题：

Thread 1:
Use CL1 to load class A (locks CL1+A)
defineClass A triggers
loadClass B (locks CL2+B)

Thread 2:
Use CL2 to load class C (locks CL2+C)
defineClass C triggers
loadClass D (locks CL1+D)

ClassLoader在初始化的时通过调用registerAsParallelCapable()方法将自己注册成为Parallel capable class loader，从而可以并发的加载class,有助于系统性能的提升。默认情况下，ClassLoader是被注册为Parallel Capable的

附JDK1.8 类ClassLoader的loadClass()方法：

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;
}
}

参考资源：

1.JDK8 ClassLoader源码

2.http://docs.oracle.com/javase/7/docs/technotes/guides/lang/cl-mt.html