Java SafeThread类中synchronized和volatile的作用？

Question

rt.jar包里有个SafeThread类，类源码如下：

final class SafeThread extends Thread {
         private volatile boolean ran = false;

         public SafeThread(Runnable target) {
             super(target);
         }

         public final void run() {
             if (Thread.currentThread() != this) {
                 throw new IllegalStateException("The run() method in a"
                     + " SafeThread cannot be called from another thread.");
             }
             synchronized (this) {
                if (!ran) {
                    ran = true;
                }
                else {
                 throw new IllegalStateException("The run() method in a"
                     + " SafeThread cannot be called more than once.");
                 }
             }
             super.run();
         }
    }

为什么加了synchronized之后，还需要给ran变量加volatile呢，如果单例模式的双检查是为了防止指令重排，但这里初始状态就已经赋值了，应该不会存在指令重排的情况了吧