java多线程--6 死锁问题 锁Lock
死锁问题
多个线程互相抱着对方需要的资源,然后形成僵持
死锁状态
package com.ssl.demo05;
public class DeadLock {
public static void main(String[] args) {
Makeup g1 = new Makeup(0,"灰");
Makeup g2 = new Makeup(1,"红");
g1.start();
g2.start();
}
}
//口红
class Lipstick{
}
//镜子
class Mirror{
}
//化妆
class Makeup extends Thread{
//资源只有一份
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
int choice; //选择
String girlName; //使用化妆品的人
Makeup(int choice,String girlName){
this.choice = choice;
this.girlName = girlName;
}
@Override
public void run() {
//化妆
try {
makeup();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
//化妆,互相持有对方的锁
private void makeup(){
if (choice==0){
synchronized (lipstick){//获得口红的锁
System.out.println(this.girlName+"获得口红的锁");
try {
Thread.sleep(1000);
} catch (Exception e) {
throw new RuntimeException(e);
}
synchronized (mirror){ //1s 后获得镜子
System.out.println(this.girlName+"获得镜子的锁");
}
}
}else {
synchronized (mirror){//获得镜子
System.out.println(this.girlName+"获得镜子的锁");
try {
Thread.sleep(2000);
} catch (Exception e) {
throw new RuntimeException(e);
}
synchronized (lipstick){ //2s 获得口红
System.out.println(this.girlName+"获得口红的锁");
}
}
}
}
}
去除死锁
不要锁中锁
package com.ssl.demo05;
public class DeadLock {
public static void main(String[] args) {
Makeup g1 = new Makeup(0,"灰");
Makeup g2 = new Makeup(1,"红");
g1.start();
g2.start();
}
}
//口红
class Lipstick{
}
//镜子
class Mirror{
}
//化妆
class Makeup extends Thread{
//资源只有一份
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
int choice; //选择
String girlName; //使用化妆品的人
Makeup(int choice,String girlName){
this.choice = choice;
this.girlName = girlName;
}
@Override
public void run() {
//化妆
try {
makeup();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
//化妆,互相持有对方的锁
private void makeup(){
if (choice==0){
synchronized (lipstick){//获得口红的锁
System.out.println(this.girlName+"获得口红的锁");
try {
Thread.sleep(1000);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
synchronized (mirror){ //1s 后获得镜子
System.out.println(this.girlName+"获得镜子的锁");
}
}else {
synchronized (mirror){//获得镜子
System.out.println(this.girlName+"获得镜子的锁");
try {
Thread.sleep(2000);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
synchronized (lipstick){ //2s 获得口红
System.out.println(this.girlName+"获得口红的锁");
}
}
}
}
产生死锁的四个必要条件
- 互斥条件: 一个资源每次只能被一个进程使用。
- 请求与保持条件:一个进程因请求资源而阻塞时,对已获得的资源保持不放。
- 不剥夺条件:进程已获得的资源,在未使用之前,不能强行剥夺。
- 循环等待条件:若干个进程之间形成一种头尾相接的循环等待资源关系。
Lock锁 JDK5开始
- 从JDK5开始,通过显示定义同步锁对象来实现同步。同步锁使用Lock对象充当。
- java.util.concurrent.locks.Lock接口是控制多个线程对共享资源进行访问的工具。锁提供了对共享资源的独占访问,每次只能有一个线程对Lock对象加锁,线程开始访问共享资源之前应先获得Lock对象。
- ReentrantLock(可重入锁)类实现了Lock,它拥有与synchronized 相同的并发性和内存语意,在实现线程安全的控制中,比较常用的是ReentrantLock,可以显式加锁、释放锁。
package com.ssl.demo05;
import java.util.concurrent.locks.ReentrantLock;
//测试Lock锁
public class TestLock {
public static void main(String[] args) {
TestLock2 testLock2 = new TestLock2();
new Thread(testLock2).start();
new Thread(testLock2).start();
new Thread(testLock2).start();
}
}
class TestLock2 implements Runnable{
int ticketNums = 10;
//定义Lock锁
private ReentrantLock lock = new ReentrantLock();
@Override
public void run() {
while(true){
try{
lock.lock(); //加锁
if (ticketNums >0){
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
System.out.println(ticketNums--);
}else{
break;
}
}finally {
//解锁
lock.unlock();
}
}
}
}
- 使用顺序
- Lock > 同步代码块 > 同步方法。