双亲委派机制

public abstract class ClassLoader {
    private final ClassLoader parent;

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // 首先，检查该类是否已经被加载
            Class<?> c = findLoadedClass(name);
            // 类没有被加载才会进入加载类的方法
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    // 存在父加载器，递归的交由父加载器
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        // 直到最上面的Bootstrap类加载器
                        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;
        }
    }
}