加锁问题，必须加锁在对象上或方法上，加在基本数据类型上无效

如下代码：
运行结果：

Thread-0 holds the locktrue
Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
at java.lang.Object.notify(Native Method)
at blockthread.DeadLock$Thread1.run(DeadLock.java:24)
main holds the lockfalse
Thread-0:99
Thread-1 holds the lockfalse
Thread-1:100

失败原因是： synchronized 只能同步对象或者方法，如果在基本数据类型上给它加锁，synchronized 无效。

package blockthread;

public class DeadLock {
    
    public static Integer i=100;
    
    public static void main(String[] args) {
        Thread1 t1=new Thread1();
        Thread2 t2=new Thread2();
        t1.start();
        t2.start();
        System.out.println(Thread.currentThread().getName()+" holds the lock"+Thread.holdsLock(i));
    }
    
    static class Thread1 extends Thread{
        
        public void run(){
            while(true){
                synchronized(i){
                    System.out.println(this.getName()+" holds the lock"+Thread.holdsLock(i));
                    if(i>0)
                    i--;
                    System.out.println(this.getName()+":"+i);
                    i.notify();
                    try {
                        i.wait();
                        System.out.println("线程1 等待");
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                //i.notify();
            }
        }
    }
    
    static class Thread2 extends Thread{
        
        public void run(){
            while(true){
                synchronized(i){
                    System.out.println(this.getName()+" holds the lock"+Thread.holdsLock(i));
                    if(i<100)
                    i++;
                    System.out.println(this.getName()+":"+i);
                    i.notify();
                    try {
                        i.wait();
                        System.out.println("线程2等待");
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                //i.notify();
            }
        }
    }

}

代码改为如下： 正常运行。

package blockthread;

public class DeadLock {
    
    
    public static void main(String[] args) {
        Instance instance=new Instance();
        Thread1 t1=new Thread1(instance);
        Thread2 t2=new Thread2(instance);
        t1.start();
        t2.start();
        System.out.println(Thread.currentThread().getName()+" holds the lock"+Thread.holdsLock(instance));
    }
    
    static class Thread1 extends Thread{
        private Instance instance;
        
        public Thread1(Instance instance){
            this.instance=instance;
        }
        
        public void run(){
            while(true){
                synchronized(instance){
                    System.out.println(this.getName()+" holds the lock"+Thread.holdsLock(instance));
                    if(instance.getI()>0){
                        int i=instance.getI();
                        i--;
                    }
                    System.out.println(this.getName()+":"+instance.getI());
                    instance.notify();
                    try {
                        instance.wait();
                        System.out.println("线程1 等待");
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                //i.notify();
            }
        }
    }
    
    static class Thread2 extends Thread{
private Instance instance;
        
        public Thread2(Instance instance){
            this.instance=instance;
        }
        public void run(){
            while(true){
                synchronized(instance){
                    System.out.println(this.getName()+" holds the lock"+Thread.holdsLock(instance));
                    if(instance.getI()<100){
                        int i=instance.getI();
                        i++;
                    }
                    System.out.println(this.getName()+":"+instance.getI());
                    instance.notify();
                    try {
                        instance.wait();
                        System.out.println("线程2等待");
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                //i.notify();
            }
        }
    }
    
    

}
class Instance{
    private int i=100;

    public int getI() {
        return i;
    }

    public void setI(int i) {
        this.i = i;
    }
}