高并发-synchronized

1.synchronized加锁

package com.bjsxt.base.sync001;

import java.util.concurrent.atomic.AtomicInteger;

/**
 * 线程安全概念：当多个线程访问某一个类（对象或方法）时，这个对象始终都能表现出正确的行为，那么这个类（对象或方法）就是线程安全的。
 * synchronized：可以在任意对象及方法上加锁，而加锁的这段代码称为"互斥区"或"临界区"
 *
 */
public class MyThread extends Thread{
	
	private int count = 5 ;
	
	//synchronized加锁
	public void run(){
		count--;
		System.out.println(this.currentThread().getName() + " count = "+ count);
	}
	
	public static void main(String[] args) {
		/**
		 * 分析：当多个线程访问myThread的run方法时，以排队的方式进行处理（这里排对是按照CPU分配的先后顺序而定的），
		 * 		一个线程想要执行synchronized修饰的方法里的代码：
		 * 		1 尝试获得锁
		 * 		2 如果拿到锁，执行synchronized代码体内容；拿不到锁，这个线程就会不断的尝试获得这把锁，直到拿到为止，
		 * 		   而且是多个线程同时去竞争这把锁。（也就是会有锁竞争的问题）
		 */
		MyThread myThread = new MyThread();
		Thread t1 = new Thread(myThread,"t1");
		Thread t2 = new Thread(myThread,"t2");
		Thread t3 = new Thread(myThread,"t3");
		Thread t4 = new Thread(myThread,"t4");
		Thread t5 = new Thread(myThread,"t5");
		t1.start();
		t2.start();
		t3.start();
		t4.start();
		t5.start();
	}
}

 未加synchronied

t1 count = 0
t3 count = 0
t2 count = 0
t4 count = 0
t5 count = 0

加synchronied

t1 count = 4
t4 count = 3
t5 count = 2
t3 count = 1
t2 count = 0

2.对象锁-同一个对象的不同方法

分析：
 t1线程先持有object对象的Lock锁，t2线程可以以异步的方式调用对象中的非synchronized修饰的方法
 t1线程先持有object对象的Lock锁，t2线程如果在这个时候调用对象中的同步（synchronized）方法则需等待，也就是同步

package com.bjsxt.base.sync003;

/**
 * 对象锁的同步和异步问题
 *
 */
public class MyObject {

	public synchronized void method1(){
		try {
			System.out.println(Thread.currentThread().getName()+"第一步");
			Thread.sleep(4000);
			System.out.println(Thread.currentThread().getName()+"第二步");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
	/** synchronized */
	public void method2(){
			System.out.println(Thread.currentThread().getName());
	}
	
	public static void main(String[] args) {
		
		final MyObject mo = new MyObject();
		
		/**
		 * 分析：
		 * t1线程先持有object对象的Lock锁，t2线程可以以异步的方式调用对象中的非synchronized修饰的方法
		 * t1线程先持有object对象的Lock锁，t2线程如果在这个时候调用对象中的同步（synchronized）方法则需等待，也就是同步
		 */
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				mo.method1();
			}
		},"t1");
		
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				mo.method2();
			}
		},"t2");
		
		t1.start();
		t2.start();
		
	}
	
}

 （1）method2不加synchronized修饰

t2
t1第一步
t1第二步

  （2）method2加synchronized修饰

t1第一步
t1第二步
t2

如果方法1和方法2是同一个事务里的话，需要都用synchronized修饰，保证事务的原子性

package com.bjsxt.base.sync004;
/**
 * 业务整体需要使用完整的synchronized，保持业务的原子性。
 *
 */
public class DirtyRead {

	private String username = "bjsxt";
	private String password = "123";
	
	public synchronized void setValue(String username, String password){
		this.username = username;
		
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		
		this.password = password;
		
		System.out.println("setValue最终结果：username = " + username + " , password = " + password);
	}
	
	public void getValue(){
		System.out.println("getValue方法得到：username = " + this.username + " , password = " + this.password);
	}
	
	
	public static void main(String[] args) throws Exception{
		
		final DirtyRead dr = new DirtyRead();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				dr.setValue("z3", "456");		
			}
		});
		t1.start();
		Thread.sleep(1000);
		
		dr.getValue();
	}
}

 主线程getValue时，t1可能已经重置了username，但还未执行后后续的重置password

getValue方法得到：username = z3 , password = 123
setValue最终结果：username = z3 , password = 456

 

3.对象锁-不同对象的同一方法

synchronized加在静态方法上的话，会变成类锁

package com.bjsxt.base.sync002;
/**
 * 关键字synchronized取得的锁都是对象锁，而不是把一段代码（方法）当做锁，
 * 所以代码中哪个线程先执行synchronized关键字的方法，哪个线程就持有该方法所属对象的锁（Lock），
 * 
 * 在静态方法上加synchronized关键字，表示锁定.class类，类一级别的锁（独占.class类）。
 *
 */
public class MultiThread {

	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 + ", num = " + num);
			
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
	//注意观察run方法输出顺序
	public static void main(String[] args) {
		
		//俩个不同的对象
		final MultiThread m1 = new MultiThread();
		final MultiThread m2 = new MultiThread();
		
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				m1.printNum("a");
			}
		});
		
		Thread t2 = new Thread(new Runnable() {
			@Override 
			public void run() {
				m2.printNum("b");
			}
		});		
		
		t1.start();
		t2.start();
		
	}
	
}

 （1）修饰普通方法

tag a, set num over!
tag b, set num over!
tag b, num = 200
tag a, num = 100

（2）修饰静态方法

tag a, set num over!
tag a, num = 100
tag b, set num over!
tag b, num = 200

4.synchronized遇到异常，锁自动释放

package com.bjsxt.base.sync005;
/**
 * synchronized异常
 *
 */
public class SyncException {

	private int i = 0;
	public synchronized void operation(){
		while(true){
			try {
				i++;
				Thread.sleep(100);
				System.out.println(Thread.currentThread().getName() + " , i = " + i);
				if(i == 2){
					//Integer.parseInt("a");
					throw new RuntimeException();
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	public static void main(String[] args) {
		
		final SyncException se = new SyncException();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				se.operation();
			}
		},"t1");
		t1.start();
	}
}

 打印 为什么退出循环了？

t1 , i = 1
t1 , i = 2
Exception in thread "t1" java.lang.RuntimeException
	at com.bjsxt.base.sync005.SyncException.operation(SyncException.java:18)
	at com.bjsxt.base.sync005.SyncException$1.run(SyncException.java:32)
	at java.lang.Thread.run(Unknown Source)

 例2

package com.bjsxt.base.sync005;
/**
 * synchronized异常
 *
 */
public class SyncException {

	private int i = 0;
	public synchronized void operation(){
		while(i<5){
			try {
				i++;
				Thread.sleep(100);
				System.out.println(Thread.currentThread().getName() + " , i = " + i);
				if(i == 2){
					//Integer.parseInt("a");
					throw new RuntimeException();
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	public static void main(String[] args) {
		
		final SyncException se = new SyncException();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				se.operation();
			}
		},"t1");
		t1.start();
//		try {
//			Thread.sleep(1000);
//		} catch (Exception e) {
//			// TODO: handle exception
//		}
		se.operation();
	}
	
	
}

 打印

main , i = 1
main , i = 2
Exception in thread "main" java.lang.RuntimeException
	at com.bjsxt.base.sync005.SyncException.operation(SyncException.java:18)
	at com.bjsxt.base.sync005.SyncException.main(SyncException.java:41)
t1 , i = 3
t1 , i = 4
t1 , i = 5

 5.锁的可重入性

（1）同一个对象的不同方法

当一个线程再次请求自己持有的对象锁的另一个临界资源时，这个情况属于重入锁，就是一个线程得到一个对象锁之后再次请求该对象锁，这种是允许的。

package com.bjsxt.base.sync005;
/**
 * synchronized的重入
 *
 */
public class SyncDubbo1 {

	public synchronized void method1(){
		System.out.println(Thread.currentThread().getName()+" "+"method1..");
		method2();
	}
	public synchronized void method2(){
		System.out.println(Thread.currentThread().getName()+" "+"method2..");
		method3();
	}
	public synchronized void method3(){
		System.out.println(Thread.currentThread().getName()+" "+"method3..");
	}
	
	public static void main(String[] args) {
		final SyncDubbo1 sd = new SyncDubbo1();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				sd.method1();
			}
		});
		t1.start();
	}
}

 （2）子类与父类

子类继承父类时，也是可以通过重入锁调用父类的同步方法

package com.bjsxt.base.sync005;
/**
 * synchronized的重入
 *
 */
public class SyncDubbo2 {

	static class Main {
		public int i = 5;
		public synchronized void operationSup(){
			try {
				i--;
				System.out.println("Main print i = " + i);
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	static class Sub extends Main {
		public synchronized void operationSub(){
			try {
				while(i > 0) {
					i--;
					System.out.println("Sub print i = " + i);
					Thread.sleep(100);		
					this.operationSup();
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	public static void main(String[] args) {
		
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				Sub sub = new Sub();
				sub.operationSub();
			}
		});
		
		t1.start();
	}
}

 6.锁对象改变的情况

（1）synchronized String、Integer等类型，当变量值发生改变时，会生成一个新对象

• String值改变，会new Stirng()，获得的是不同的对象锁，可以用intern()方法

这篇文章有介绍：https://www.cnblogs.com/xrq730/p/6662232.html

• Integer类型自动装箱和解箱会生成一个新的对象

package com.bjsxt.base.sync006;
/**
 * 锁对象的改变问题
 *synchronized代码块对字符串的锁，注意String常量池的缓存功能
 */
public class ChangeLock {

	private String lock = "lock";
	
	private void method(){
　　　　　　　　　　//new String("字符串常量") 
　　　　　　　　　　//synchronized ("字符串常量") {
		synchronized (lock) {
			try {
				System.out.println("当前线程 : "  + Thread.currentThread().getName() + "开始");
				lock = "change lock";
				Thread.sleep(2000);
				System.out.println("当前线程 : "  + Thread.currentThread().getName() + "结束");
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	public static void main(String[] args) {
	
		final ChangeLock changeLock = new ChangeLock();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				changeLock.method();
			}
		},"t1");
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				changeLock.method();
			}
		},"t2");
		t1.start();
		try {
			Thread.sleep(100);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		t2.start();
	}
	
}

 打印

当前线程 : t1开始
当前线程 : t2开始
当前线程 : t1结束
当前线程 : t2结束

 

（2）同一对象的属性值修改时，不会影响锁

package com.bjsxt.base.sync006;
/**
 * 同一对象属性的修改不会影响锁的情况
 *
 */
public class ModifyLock {
	
	private String name ;
	private int age ;
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	
	public synchronized void changeAttributte(String name, int age) {
		try {
			System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 开始");
			this.setName(name);
			this.setAge(age);
			
			System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 修改对象内容为： " 
					+ this.getName() + ", " + this.getAge());
			
			Thread.sleep(2000);
			System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 结束");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
	public static void main(String[] args) {
		final ModifyLock modifyLock = new ModifyLock();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				modifyLock.changeAttributte("张三", 20);
			}
		},"t1");
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				modifyLock.changeAttributte("李四", 21);
			}
		},"t2");
		
		t1.start();
//		try {
//			Thread.sleep(100);
//		} catch (InterruptedException e) {
//			e.printStackTrace();
//		}
		t2.start();
	}
	
}

 7.synchronized 锁对象的不同方式

package com.bjsxt.base.sync006;

/**
 * 使用synchronized代码块加锁,比较灵活
 */
public class ObjectLock {

	public void method1(){
		synchronized (this) {	//对象锁
			try {
				System.out.println("do method1..");
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	public void method2(){		//类锁
		synchronized (ObjectLock.class) {
			try {
				System.out.println("do method2..");
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	private Object lock = new Object();
	public void method3(){		//任何对象锁
		synchronized (lock) {
			try {
				System.out.println("do method3..");
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	
	public static void main(String[] args) {
		
		final ObjectLock objLock = new ObjectLock();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				objLock.method1();
			}
		});
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				objLock.method2();
			}
		});
		Thread t3 = new Thread(new Runnable() {
			@Override
			public void run() {
				objLock.method3();
			}
		});
		
		t1.start();
		t2.start();
		t3.start();
	}
	
}

 8.死锁

package com.bjsxt.base.sync006;

/**
 * 死锁问题，在设计程序时就应该避免双方相互持有对方的锁的情况
 */
public class DeadLock implements Runnable{

	private String tag;
	private static Object lock1 = new Object();
	private static Object lock2 = new Object();
	
	public void setTag(String tag){
		this.tag = tag;
	}
	
	@Override
	public void run() {
		if(tag.equals("a")){
			synchronized (lock1) {
				try {
					System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 进入lock1执行");
					Thread.sleep(2000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				synchronized (lock2) {
					System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 进入lock2执行");
				}
			}
		}
		if(tag.equals("b")){
			synchronized (lock2) {
				try {
					System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 进入lock2执行");
					Thread.sleep(2000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				synchronized (lock1) {
					System.out.println("当前线程 : "  + Thread.currentThread().getName() + " 进入lock1执行");
				}
			}
		}
	}
	
	public static void main(String[] args) {
		
		DeadLock d1 = new DeadLock();
		d1.setTag("a");
		DeadLock d2 = new DeadLock();
		d2.setTag("b");
		 
		Thread t1 = new Thread(d1, "t1");
		Thread t2 = new Thread(d2, "t2");
		 
		t1.start();
		try {
			Thread.sleep(500);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		t2.start();
	}
}

 9.尽量减小锁的粒度

package com.bjsxt.base.sync006;

/**
 * 使用synchronized代码块减小锁的粒度，提高性能
 *
 */
public class Optimize {

	public void doLongTimeTask(){
		try {
			
			System.out.println("当前线程开始：" + Thread.currentThread().getName() + 
					", 正在执行一个较长时间的业务操作，其内容不需要同步");
			Thread.sleep(2000);
			
			synchronized(this){
				System.out.println("当前线程：" + Thread.currentThread().getName() + 
					", 执行同步代码块，对其同步变量进行操作");
				Thread.sleep(1000);
			}
			System.out.println("当前线程结束：" + Thread.currentThread().getName() +
					", 执行完毕");
			
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
	public static void main(String[] args) {
		final Optimize otz = new Optimize();
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				otz.doLongTimeTask();
			}
		},"t1");
		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				otz.doLongTimeTask();
			}
		},"t2");
		t1.start();
		t2.start();
		
	}
	
}