多线程基础学习第二篇（多个线程多把锁）

一时失言乱红尘 2022-09-30 12:59 180阅读 0赞

多个线程同时竞争多把锁（这是对象上的锁,因为每个对象都有一把锁，所以彼此之间不存在关系）

/** * @version V1.0 * @Description: 多个线程同时竞争多把锁 * 在静态方法上加synchronize关键字，表示锁定.class类，类一级别的锁(独占.class类) * @Modified By:Ming Created in 14:39 2017/3/22 */
    public class MyThread1 { 
    
        private  int num = 0;
    
        /** * static **/
        public  synchronized void printNum(String tag) {
            try {
                if (tag.equals("a")) {
                    num = 100;
                    System.out.println("tag a, set num over!" );
                    Thread.sleep(1000);
                } else {
                    num = 200;
                    System.out.println("tag b, set num over!");
                }
                System.out.println("tag " + tag + ", set num over!" + num);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    
        //注意观察run方法的输出顺序
        public static void main(String[] args) {
    
            //俩个不同的对象
            final MyThread1 m1 = new MyThread1();
            final MyThread1 m2 = new MyThread1();
    
            Thread t1 = new Thread(new Runnable() {
    
                public void run() {
                    m1.printNum("a");
                }
            });
    
            Thread t2 = new Thread(new Runnable() {
                public void run() {
                    m2.printNum("b");
                }
            });
    
            t1.start();
            t2.start();
        }
    }
    
    tag a, set num over!
    tag b, set num over!
    tag b, set num over!200
    tag a, set num over!100

关键字synchronized取得的锁都是对象锁，而不是把一段代码(方法)当作锁，所以示例中的哪个线程先执行synchronized关键字的方法，哪个线程就持有该方法所属对象的锁（lock）,如果是多个线程，那么线程获得的就是多个不同的锁，他们互不影响（但是有一种情况是相同的锁，即在静态方法上添加synchronized关键字，表示锁定.class,类一级别的锁，独占.class类）

（如果想要他们去竞争同一把锁的）加上静态修饰，变对象锁为类级别的锁

/** * @version V1.0 * @Description: 多个线程同时竞争多把锁 * 在静态方法上加synchronize关键字，表示锁定.class类，类一级别的锁(独占.class类) * @Modified By:Ming Created in 14:39 2017/3/22 */
    public class MyThread1 { 
    
        private static int num = 0;
    
        /** * static **/
        public static synchronized void printNum(String tag) {
            try {
                if (tag.equals("a")) {
                    num = 100;
                    System.out.println("tag a, set num over!" );
                    Thread.sleep(1000);
                } else {
                    num = 200;
                    System.out.println("tag b, set num over!");
                }
                System.out.println("tag " + tag + ", set num over!" + num);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    
        //注意观察run方法的输出顺序
        public static void main(String[] args) {
    
            //俩个不同的对象
            final MyThread1 m1 = new MyThread1();
            final MyThread1 m2 = new MyThread1();
    
            Thread t1 = new Thread(new Runnable() {
    
                public void run() {
                    m1.printNum("a");
                }
            });
    
            Thread t2 = new Thread(new Runnable() {
                public void run() {
                    m2.printNum("b");
                }
            });
    
            t1.start();
            t2.start();
        }
    }
    
    tag b, set num over!
    tag b, set num over!200
    tag a, set num over!
    tag a, set num over!100