一、线程安全概述

1. 什么是线程安全问题？

当多个线程共享同一个全局变量，做写的操作时（即修改该全局变量），可能会受到其他的线程干扰，发生线程安全问题。
eg：

public class Thread01  implements  Runnable{//定义一个全局变量private static Integer count = 100;@Overridepublic void run() {while (count >1){cal();}}private void cal(){try {Thread.sleep(30);} catch (InterruptedException e) {e.printStackTrace();}count--;System.out.println(Thread.currentThread().getName()+"--"+count);}public static void main(String[] args) {Thread01 thread01 = new Thread01();Thread thread1 = new Thread(thread01);Thread thread2 = new Thread(thread01);thread1.start();thread2.start();}
}

运行后打印结果出现了线程安全问题，如下图：

2. 多线程如何解决线程安全问题（多线程如何实现同步）

当多个线程共享同一个全局变量时，将可能会发生线程安全的代码上锁，保证只有拿到锁的线程才可以执行，没有拿到锁的线程不可以执行，需要阻塞等待。
（1）使用synchronized锁；
（2）使用Lock锁 ，需要自己实现锁的升级过程，底层是基于aqs实现；
（3）使用Threadlocal，需要注意内存泄漏的问题。
（4）原子类CAS 非阻塞式。

二、synchronized锁的基本用法

1. 修饰代码块
   修饰代码块，指定加锁对象，对指定对象加锁，执行该代码块前要获得指定对象的锁。
   eg：

public class Thread01 implements Runnable{private static Integer count = 100;@Overridepublic void run() {while (count > 1) {test();}}private void test() {synchronized (this) {try {Thread.sleep(20);} catch (InterruptedException e) {e.printStackTrace();}count--;System.out.println(Thread.currentThread().getName() + "," + count);}}public static void main(String[] args) {Thread01 thread01 = new Thread01();Thread thread1 = new Thread(thread01);Thread thread2 = new Thread(thread01);thread1.start();thread2.start();}
}

2. 修饰实例方法
   修饰实例方法，作用于当前实例加锁，进入同步代码前要获得当前实例的锁，在实例方法上默认加上synchronized 默认使用this锁。
   eg：

public class Thread02 implements Runnable {private static Integer count = 100;@Overridepublic void run() {while (count > 1) {test();}}private synchronized void test() {try {Thread.sleep(10);} catch (Exception e) {}count--;System.out.println(Thread.currentThread().getName() + "," + count);}public static void main(String[] args) {Thread02 thread02 = new Thread02();Thread thread1 = new Thread(thread02);Thread thread2 = new Thread(thread02);thread1.start();thread2.start();}
}

3. 修饰静态方法
   修饰静态方法，作用于当前类对象加锁，进入同步代码前要获得当前类对象的锁，默认使用当前类的 类名.class 锁。
   eg：

public class Thread03 implements Runnable{private static Integer count = 100;private static String lock = "lock";@Overridepublic void run() {while (count > 1) {test();}}private static void test() {synchronized (Thread03.class) {try {Thread.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}count--;System.out.println(Thread.currentThread().getName() + "," + count);}}public static void main(String[] args) {Thread03 Thread031 = new Thread03();Thread03 Thread032 = new Thread03();Thread thread1 = new Thread(Thread031);Thread thread2 = new Thread(Thread032);thread1.start();thread2.start();}
}

4. synchronized死锁问题
   在使用synchronized时需要注意 synchronized锁嵌套的问题，避免死锁的问题发生。
   eg：

public class ThreadDeadlock implements Runnable{private int count = 1;private String lock = "lock";@Overridepublic void run() {while (true) {count++;if (count % 2 == 0) {// 线程1需要先获取到自定义对象的lock锁，执行test1方法需要再获取this锁;线程2需要先获取this锁，执行test2方法再获取lock锁synchronized (lock) {test1();}} else {synchronized (this) {test2();}}}}public synchronized void test1() {System.out.println(Thread.currentThread().getName() + ",test1方法");}public void test2() {synchronized (lock) {System.out.println(Thread.currentThread().getName() + ",test2方法");}}public static void main(String[] args) {ThreadDeadlock threadDeadlock = new ThreadDeadlock();Thread thread1 = new Thread(threadDeadlock);Thread thread2 = new Thread(threadDeadlock);thread1.start();thread2.start();}
}

5. springmvc 接口中使用
   Spring MVC 的Controller默认是单例的，需要注意线程安全问题。
   eg：

@RestController
public class ThreadService {private int count = 0;@RequestMapping("/test")public synchronized String count() {try {System.out.println(">count<" + count++);Thread.sleep(3000);} catch (Exception e) {e.printStackTrace();}return "count";}
}

6. 多线程线程之间通讯

等待/通知机制：
等待/通知的相关方法是任意Java对象都具备的，因为这些方法被定义在所有对象的超类java.lang.Object上，方法如下：
（1）notify()：通知一个在对象上等待的线程，使其从main()方法返回，而返回的前提是该线程获取到了对象的锁；
（2）notifyAll()：通知所有等待在该对象的线程；
（3）wait()：调用该方法的线程进入WAITING状态，只有等待其他线程的通知或者被中断，才会返回。需要注意调用wait()方法后，会释放对象的锁 。

注意：wait()、notify()和notifyAll()方法要与synchronized一起使用。

eg：

public class Thread02 {private  Object objectLock = new Object();public static void main(String[] args) throws InterruptedException {new Thread02().print();}public void print() throws InterruptedException {new Thread(new Runnable() {@Overridepublic void run() {synchronized (objectLock){System.out.println(Thread.currentThread().getName()+"---1---");try {objectLock.wait();} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName()+"---2---");}}}).start();Thread.sleep(3000);synchronized (objectLock){objectLock.notify();}}
}

7. 多线程通讯实现生产者与消费者
   eg：

public class Thread03 {class Student {public String name;public char sex;public boolean flag = true;//true:输入，false：输出}class InputThread extends Thread {private Student student;public InputThread(Student student) {this.student = student;}@Overridepublic void run() {int count = 0;while (true) {synchronized (student) {if (!student.flag) {try {student.wait();} catch (InterruptedException e) {e.printStackTrace();}}if (count == 0) {student.name = "毛毛";student.sex = '男';} else {student.name = "天天";student.sex = '女';}count = (count + 1) % 2;student.flag = false;student.notify();}}}}class PrintThread extends Thread {private Student student;public PrintThread(Student student) {this.student = student;}@Overridepublic void run() {while (true) {synchronized (student) {if (student.flag) {try {student.wait();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("姓名：" + student.name + "，性别：" + student.sex);student.flag = true;student.notify();}}}}public void print() {Student student = new Student();InputThread inputThread = new InputThread(student);PrintThread printThread = new PrintThread(student);inputThread.start();printThread.start();}public static void main(String[] args) {new Thread03().print();}}