前言
本系列记录Java从入门开始的知识点，本文介绍线程同步：什么是线程同步、线程不安全的例子、同步块、JUC、死锁和Lock锁。
一、什么是线程同步
- 在处理多线程问题时，多个线程访问同一个对象，并且某些线程还想修改这个对象，这时就需要线程同步。线程同步是一种等待机制，多个需要同时访问此对象的线程进入这个
对象等待池形成对列，等待前面的线程使用完毕，下一个线程再使用。 - 为了实现线程的安全性，需要使用对列+锁。
- 由于同一进程的多个线程共享同一块存储空间，在带来方便的同时，也带来了访问冲突问题，为了保证数据在方法中被访问时的正确性，在访问时加入锁机制。当一个线程获得对象的排它锁，独占资源，其他线程必须等待，使用后释放锁即可。
- 线程同步存在的问题：
（1）一个线程持有锁会导致其他所有需要此锁的线程挂起；
（2）在多线程竞争下，加锁，释放锁，会导致比较多的上下文切换和调度延时，引起性能问题；
（3）如果一个优先级高的线程等待一个优先级低的线程释放锁，会导致优先级倒置，引起性能问题；
二、线程不安全的例子
- 不安全抢票：
//不安全的买票
public class UnsafeBuyTicket {
public static void main(String[] args) {
BuyTicket buyTicket = new BuyTicket();
new Thread(buyTicket,"小红").start();
new Thread(buyTicket,"小明").start();
}
}
class BuyTicket implements Runnable{
private int ticketNums = 10;
boolean flag = true;
@Override
public void run() {
while (flag) {
buy();
}
}
public synchronized void buy(){
if (ticketNums <= 0) {
flag = false;
return;
}
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "拿到了第" + ticketNums-- + "张票");
}
}
- 不安全取钱：
package Thread.syn;
import oop.demo08.Action;
public class UnsafeBank {
public static void main(String[] args) {
Account account = new Account(100,"结婚基金");
Drawing you = new Drawing(account, 50, "你");
Drawing boyFriend = new Drawing(account, 100, "boyFriend");
you.start();
boyFriend.start();
}
}
//账户
class Account{
int money;
String name;
public Account(int money,String name) {
this.money = money;
this.name = name;
}
}
class Drawing extends Thread{
Account account;
int drawingMoney;
int nowMoney;
String name;
public Drawing(Account account,int drawingMoney,String name){
super(name);
this.account = account;
this.drawingMoney = drawingMoney;
}
@Override
public void run() {
if(account.money - drawingMoney < 0){
System.out.println(Thread.currentThread().getName()+"钱不够，取不了");
return;
}
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
account.money = account.money - drawingMoney;
nowMoney = nowMoney + drawingMoney;
System.out.println(account.name+"余额为："+account.money);
System.out.println(this.getName()+"手里的钱为："+nowMoney);
}
}
- 不安全列表：
import java.util.ArrayList;
import java.util.List;
public class UnsafeList {
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
for (int i = 0; i < 10000; i++) {
new Thread(()->{
list.add(Thread.currentThread().getName());
}).start();
}
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(list.size());
}
}
三、同步块
- 解决不安全买票：
public synchronized void buy(){
...}
2. 解决不安全取钱
public void run() {
synchronized (account) {
if (account.money - drawingMoney < 0) {
System.out.println(Thread.currentThread().getName() + "钱不够，取不了");
return;
}
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
account.money = account.money - drawingMoney;
nowMoney = nowMoney + drawingMoney;
System.out.println(account.name + "余额为：" + account.money);
System.out.println(this.getName() + "手里的钱为：" + nowMoney);
}
}
- 解决不安全列表
new Thread(()->{
synchronized (list){
list.add(Thread.currentThread().getName());
}
}).start();
四、扩展：JUC
package Thread.syn;
import java.util.concurrent.CopyOnWriteArrayList;
//测试JUC安全
public class TestJUC {
public static void main(String[] args) {
CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<String>();
for (int i = 0; i < 10000; i++) {
new Thread(()->{
list.add(Thread.currentThread().getName());
}).start();
}
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(list.size());
}
}
五、死锁
死锁就是两个对象手中都持有对方需要的资源，谁都不先放手，就会产生死锁。
package Thread.syn;
public class DeadLock {
public static void main(String[] args) {
Makeup g1 = new Makeup(0,"灰姑凉");
Makeup g2 = new Makeup(1,"白雪公主");
g1.start();
g2.start();
}
}
class Lipstick{
}
class Mirror{
}
class Makeup extends Thread{
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
int choice;
String girlName;
Makeup(int choice,String girlName){
this.choice = choice;
this.girlName = girlName;
}
@Override
public void run() {
try {
makeup();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void makeup() throws InterruptedException {
if(choice == 0){
synchronized (lipstick){
System.out.println(this.girlName+"获得口红的锁");
Thread.sleep(1000);
synchronized (mirror){
System.out.println(this.girlName+"获得镜子的锁");
}
}
}else{
synchronized (mirror){
System.out.println(this.girlName+"获得镜子的锁");
Thread.sleep(1000);
synchronized (lipstick){
System.out.println(this.girlName+"获得口红的锁");
}
}
}
}
}
六、Lock锁
- Lock锁的使用方式
- 解决不安全买票：
package Thread.syn;
import java.util.concurrent.locks.ReentrantLock;
public class TestLock {
public static void main(String[] args) {
TestLock2 testLock2 = new TestLock2();
new Thread(testLock2,"小红").start();
new Thread(testLock2,"小明").start();
new Thread(testLock2,"小蓝").start();
}
}
class TestLock2 implements Runnable{
int ticketNums = 100;
private final ReentrantLock lock = new ReentrantLock();
@Override
public void run() {
while (true){
try {
lock.lock();
if(ticketNums<=0){
break;}
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+"拿到第"+ticketNums--+"张票");
} finally {
lock.unlock();
}
}
}
}
- synchronized和Lock的对比
