Java多线程下解决数据安全问题 Java多线程下解决数据安全问题

同步代码块
同步方法
lock锁

同步代码块基本语句

synchronized (任意对象) {
操作共享代码
}

代码示例

public class SellTicket implements Runnable {private int tickets = 100; private Object object = new Object(); @Overridepublic void run() {while (true) {synchronized (object) {if (tickets > 0) {try {Thread.sleep(100); } catch (InterruptedException e) {e.printStackTrace(); }System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); tickets--; }}}}public static void main(String[] args) {SellTicket sellTicket = new SellTicket(); Thread thread1 = new Thread(sellTicket, "窗口1"); Thread thread2 = new Thread(sellTicket, "窗口2"); Thread thread3 = new Thread(sellTicket, "窗口3"); thread1.start(); thread2.start(); thread3.start(); }}

优缺点：

解决了多线程的数据安全问题
多线程时，每个线程都会判断同步上的锁，耗费资源，降低了程序的运行效率

同步方法同步方法：将synchronized关键字加到方法上

格式: 修饰符 synchronized 返回值类型方法名(){ }
同步方法的锁对象是this

同步静态方法，就是把synchronized关键字加到静态方法上

格式: 修饰符 static synchronized 返回值类型方法名(){ }
同步静态方法的锁对象是类名.class

代码示例

public class SellTicket implements Runnable {//private int tickets = 100; private static int tickets = 100; private Object object = new Object(); private int x = 0; @Overridepublic void run() {while (true) {if (x % 2 == 0) {//synchronized (object) {//synchronized (this) {synchronized (SellTicket.class) {if (tickets > 0) {try {Thread.sleep(100); } catch (InterruptedException e) {e.printStackTrace(); }System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); tickets--; }}} else {//synchronized (object) {//if (tickets > 0) {//try {//Thread.sleep(100); //} catch (InterruptedException e) {//e.printStackTrace(); //}//System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); //tickets--; //}//}sellTicket(); }x++; }}//private void sellTicket(){//synchronized (object) {//if (tickets > 0) {//try {//Thread.sleep(100); //} catch (InterruptedException e) {//e.printStackTrace(); //}//System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); //tickets--; //}//}//}//private synchronized void sellTicket(){//if (tickets > 0) {//try {//Thread.sleep(100); //} catch (InterruptedException e) {//e.printStackTrace(); //}//System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); //tickets--; //}private static synchronized void sellTicket(){if (tickets > 0) {try {Thread.sleep(100); } catch (InterruptedException e) {e.printStackTrace(); }System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); tickets--; }}public static void main(String[] args) {SellTicket sellTicket = new SellTicket(); Thread thread1 = new Thread(sellTicket, "窗口1"); Thread thread2 = new Thread(sellTicket, "窗口2"); Thread thread3 = new Thread(sellTicket, "窗口3"); thread1.start(); thread2.start(); thread3.start(); }}

lock锁 lock实现提供比使用synchronized方法和语句可获得更广泛的操作

void lock()获得锁
void unlock()释放

lock是接口不能直接实例化，采用实现类实例化ReentrantLock

import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class SellTicket implements Runnable {private int tickets = 100; private Object object = new Object(); private Lock lock = new ReentrantLock(); @Overridepublic void run() {while (true) {try {lock.lock(); if (tickets > 0) {try {Thread.sleep(100); } catch (InterruptedException e) {e.printStackTrace(); }System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "张票"); tickets--; }} finally {lock.unlock(); }}}public static void main(String[] args) {SellTicket sellTicket = new SellTicket(); Thread thread1 = new Thread(sellTicket, "窗口1"); Thread thread2 = new Thread(sellTicket, "窗口2"); Thread thread3 = new Thread(sellTicket, "窗口3"); thread1.start(); thread2.start(); thread3.start(); }

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