类加载
- 核心:
loadClass(String name)
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
//锁定，防止重复加载
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
//是否已经加载
if (c == null) {
long t0 = System.nanoTime();
try {
//优先父类
if (parent != null) {
c = parent.loadClass(name, false);
}
//若父类为null，则使用BootstrapClassLoader
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);
//启用自己的，开始记录消耗时间，并且加一找到的类记录
// this is the defining class loader; record the stats
PerfCounter.getParentDelegationTime().addTime(t1 - t0);
PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
PerfCounter.getFindClasses().increment();
}
}
//若已加载的，则直接返回
if (resolve) {
resolveClass(c);
}
return c;
}
}
- 若loadClass()失败则启用findClass()而这也是用户所应该复写的方法
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
双亲委派机制[Parents Delegation Model]
这里我觉得理解上时不能纠结于双亲，感觉这里应该是翻译不当。毕竟在加载时首先会委托给父辈s【表多个父辈】，eg：用户自定义->应用类->扩展类->启动类
而不是所谓的“双亲”委派，有时面试还会问为什么有双亲？😄
大部分使用双亲委派，还有很多不使用的场景JDK1.2前的findClass()、OSGI、JNDI的SPI接口