JAVA深化篇_32—— 线程使用之线程同步synchronized语法结构【附有详细说明及代码】
线程同步
什么是线程同步
同步问题的提出
现实生活中，我们会遇到“同一个资源，多个人都想使用”的问题。 比如：教室里，只有一台电脑，多个人都想使用。天然的解决办法就是，在电脑旁边，大家排队。前一人使用完后，后一人再使用。
线程同步的概念
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。 这时候，我们就需要用到“线程同步”。 线程同步其实就是一种等待机制，多个需要同时访问此对象的线程进入这个对象的等待池形成队列，等待前面的线程使用完毕后，下一个线程再使用。
线程冲突案例演示
我们以银行取款经典案例来演示线程冲突现象。
银行取钱的基本流程基本上可以分为如下几个步骤。
（1）用户输入账户、密码，系统判断用户的账户、密码是否匹配。
（2）用户输入取款金额
（3）系统判断账户余额是否大于或等于取款金额
（4）如果余额大于或等于取款金额，则取钱成功；如果余额小于取款金额，则取钱失败。
/**
* 账户类
*/
class Account{
//账号
private String accountNo;
//账户的余额
private double balance;
public Account() {
}
public Account(String accountNo, double balance) {
this.accountNo = accountNo;
this.balance = balance;
}
public String getAccountNo() {
return accountNo;
}
public void setAccountNo(String accountNo) {
this.accountNo = accountNo;
}
public double getBalance() {
return balance;
}
public void setBalance(double balance) {
this.balance = balance;
}
}
/**
* 取款线程
*/
class DrawThread implements Runnable{
//账户对象
private Account account;
//取款金额
private double drawMoney;
public DrawThread(Account account,double drawMoney){
this.account = account;
this.drawMoney = drawMoney;
}
/**
* 取款线程
*/
@Override
public void run() {
//判断当前账户余额是否大于或等于取款金额
if(this.account.getBalance() >= this.drawMoney){
System.out.println(Thread.currentThread().getName()+" 取钱成功！吐出钞票："+this.drawMoney);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//更新账户余额
this.account.setBalance(this.account.getBalance()- this.drawMoney);
System.out.println("\t 余额为："+this.account.getBalance());
}else{
System.out.println(Thread.currentThread().getName()+" 取钱失败，余额不足");
}
}
}
public class TestDrawMoneyThread {
public static void main(String[] args) {
Account account = new Account("1234",1000);
new Thread(new DrawThread(account,800),"老公").start();
new Thread(new DrawThread(account,800),"老婆").start();
}
}
实现线程同步
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，也带来了访问冲突的问题。Java语言提供了专门机制以解决这种冲突，有效避免了同一个数据对象被多个线程同时访问造成的这种问题。这套机制就是synchronized关键字。
synchronized语法结构：
synchronized(锁对象){
同步代码
}
synchronized关键字使用时需要考虑的问题：
- 需要对那部分的代码在执行时具有线程互斥的能力(线程互斥：并行变串行)。
- 需要对哪些线程中的代码具有互斥能力(通过synchronized锁对象来决定)。
它包括两种用法：
synchronized 方法和 synchronized 块。
-
synchronized 方法
通过在方法声明中加入 synchronized关键字来声明，语法如下：
public synchronized void accessVal(int newVal);
-
synchronized 在方法声明时使用：放在访问控制符(public)之前或之后。这时同一个对象下synchronized方法在多线程中执行时，该方法是同步的，即一次只能有一个线程进入该方法，其他线程要想在此时调用该方法，只能排队等候，当前线程(就是在synchronized方法内部的线程)执行完该方法后，别的线程才能进入。
-
synchronized块
synchronized 方法的缺陷：若将一个大的方法声明为synchronized 将会大大影响效率。
Java 为我们提供了更好的解决办法，那就是 synchronized 块。 块可以让我们精确地控制到具体的“成员变量”，缩小同步的范围，提高效率。
修改线程冲突案例演示
/**
* 账户类
*/
class Account{
//账号
private String accountNO;
//账户余额
private double balance;
public Account() {
}
public Account(String accountNO, double balance) {
this.accountNO = accountNO;
this.balance = balance;
}
public String getAccountNO() {
return accountNO;
}
public void setAccountNO(String accountNO) {
this.accountNO = accountNO;
}
public double getBalance() {
return balance;
}
public void setBalance(double balance) {
this.balance = balance;
}
}
/**
* 取款线程
*/
class DrawThread implements Runnable{
//账户对象
private Account account;
//取款金额
private double drawMoney;
public DrawThread(){
}
public DrawThread(Account account,double drawMoney){
this.account = account;
this.drawMoney = drawMoney;
}
/**
* 取款线程体
*/
@Override
public void run() {
synchronized (this.account){
//判断当前账户余额是否大于或等于取款金额
if(this.account.getBalance() >= this.drawMoney){
System.out.println(Thread.currentThread().getName()+" 取钱成功！突出钞票"+this.drawMoney);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//更新账户余额
this.account.setBalance(this.account.getBalance() - this.drawMoney);
System.out.println("\t 余额为："+this.account.getBalance());
}else{
System.out.println(Thread.currentThread().getName()+" 取钱失败，余额不足");
}
}
}
}
public class TestDrawMoneyThread {
public static void main(String[] args) {
Account account = new Account("1234",1000);
new Thread(new DrawThread(account,800),"老公").start();
new Thread(new DrawThread(account,800),"老婆").start();
}
}
线程同步的使用
使用this作为线程对象锁
语法结构：
synchronized(this){
//同步代码
}
或
public synchronized void accessVal(int newVal){
//同步代码
}
/**
* 定义程序员类
*/
class Programmer{
private String name;
public Programmer(String name){
this.name = name;
}
/**
* 打开电脑
*/
synchronized public void computer(){
try {
System.out.println(this.name + " 接通电源");
Thread.sleep(500);
System.out.println(this.name + " 按开机按键");
Thread.sleep(500);
System.out.println(this.name + " 系统启动中");
Thread.sleep(500);
System.out.println(this.name + " 系统启动成功");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 编码
*/
synchronized public void coding(){
try {
System.out.println(this.name + " 双击Idea");
Thread.sleep(500);
System.out.println(this.name + " Idea启动完毕");
Thread.sleep(500);
System.out.println(this.name + " 开开心心的写代码");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/**
* 打开电脑的工作线程
*/
class Working1 extends Thread{
private Programmer p;
public Working1(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.computer();
}
}
/**
* 编写代码的工作线程
*/
class Working2 extends Thread{
private Programmer p;
public Working2(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.coding();
}
}
public class TestSyncThread {
public static void main(String[] args) {
Programmer p = new Programmer("张三");
new Working1(p).start();
new Working2(p).start();
}
}
使用字符串作为线程对象锁
语法结构：
synchronized(“字符串”){
//同步代码
}
/**
* 定义程序员类
*/
class Programmer{
private String name;
public Programmer(String name){
this.name = name;
}
/**
* 打开电脑
*/
synchronized public void computer(){
try {
System.out.println(this.name + " 接通电源");
Thread.sleep(500);
System.out.println(this.name + " 按开机按键");
Thread.sleep(500);
System.out.println(this.name + " 系统启动中");
Thread.sleep(500);
System.out.println(this.name + " 系统启动成功");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 编码
*/
synchronized public void coding(){
try {
System.out.println(this.name + " 双击Idea");
Thread.sleep(500);
System.out.println(this.name + " Idea启动完毕");
Thread.sleep(500);
System.out.println(this.name + " 开开心心的写代码");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 去卫生间
*/
public void wc(){
synchronized ("suibian") {
try {
System.out.println(this.name + " 打开卫生间门");
Thread.sleep(500);
System.out.println(this.name + " 开始排泄");
Thread.sleep(500);
System.out.println(this.name + " 冲水");
Thread.sleep(500);
System.out.println(this.name + " 离开卫生间");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
/**
* 打开电脑的工作线程
*/
class Working1 extends Thread{
private Programmer p;
public Working1(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.computer();
}
}
/**
* 编写代码的工作线程
*/
class Working2 extends Thread{
private Programmer p;
public Working2(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.coding();
}
}
/**
* 去卫生间的线程
*/
class WC extends Thread{
private Programmer p;
public WC(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.wc();
}
}
public class TestSyncThread {
public static void main(String[] args) {
Programmer p = new Programmer("张三");
Programmer p1 = new Programmer("李四");
Programmer p2 = new Programmer("王五");
new WC(p).start();
new WC(p1).start();
new WC(p2).start();
}
}
用Class作为线程对象锁
语法结构：
synchronized(XX.class){
//同步代码
}
或
synchronized public static void accessVal()
/**
* 定义销售员工类
*/
class Sale{
private String name;
public Sale(String name){
this.name = name;
}
/**
* 领取奖金
*/
synchronized public static void money(){
try {
System.out.println(Thread.currentThread().getName() + " 被领导表扬");
Thread.sleep(500);
System.out.println(Thread.currentThread().getName() + " 拿钱");
Thread.sleep(500);
System.out.println(Thread.currentThread().getName() + " 对公司表示感谢");
Thread.sleep(500);
System.out.println(Thread.currentThread().getName() + " 开开心心的拿钱走人");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Programmer{
private String name;
public Programmer(String name){
this.name = name;
}
/**
* 打开电脑
*/
synchronized public void computer(){
try {
System.out.println(this.name + " 接通电源");
Thread.sleep(500);
System.out.println(this.name + " 按开机按键");
Thread.sleep(500);
System.out.println(this.name + " 系统启动中");
Thread.sleep(500);
System.out.println(this.name + " 系统启动成功");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 编码
*/
synchronized public void coding(){
try {
System.out.println(this.name + " 双击Idea");
Thread.sleep(500);
System.out.println(this.name + " Idea启动完毕");
Thread.sleep(500);
System.out.println(this.name + " 开开心心的写代码");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 去卫生间
*/
public void wc(){
synchronized ("suibian") {
try {
System.out.println(this.name + " 打开卫生间门");
Thread.sleep(500);
System.out.println(this.name + " 开始排泄");
Thread.sleep(500);
System.out.println(this.name + " 冲水");
Thread.sleep(500);
System.out.println(this.name + " 离开卫生间");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/**
* 领取奖金
*/
public void money(){
synchronized (Programmer.class) {
try {
System.out.println(this.name + " 被领导表扬");
Thread.sleep(500);
System.out.println(this.name + " 拿钱");
Thread.sleep(500);
System.out.println(this.name + " 对公司表示感谢");
Thread.sleep(500);
System.out.println(this.name + " 开开心心的拿钱走人");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
/**
* 打开电脑的工作线程
*/
class Working1 extends Thread{
private Programmer p;
public Working1(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.computer();
}
}
/**
* 编写代码的工作线程
*/
class Working2 extends Thread{
private Programmer p;
public Working2(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.coding();
}
}
/**
* 去卫生间的线程
*/
class WC extends Thread{
private Programmer p;
public WC(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.wc();
}
}
/**
* 程序员领取奖金
*/
class ProgrammerMoney extends Thread{
private Programmer p;
public ProgrammerMoney(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.money();
}
}
/**
* 销售部门领取奖金
*/
class SaleMoney extends Thread{
private Sale p;
public SaleMoneyThread(Sale p){
this.p = p;
}
@Override
public void run() {
this.p.money();
}
}
public class TestSyncThread {
public static void main(String[] args) {
/* Programmer p = new Programmer("张三");
Programmer p1 = new Programmer("李四");
new ProgrammerMoney(p).start();
new ProgrammerMoney(p1).start();*/
Sale s = new Sale("张晓丽");
Sale s1 = new Sale("王晓红");
new SaleMoney(s).start();
new SaleMoney(s1).start();
}
}
使用自定义对象作为线程对象锁
语法结构：
synchronized(自定义对象){
//同步代码
}
/**
* 定义销售员工类
*/
class Sale{
private String name;
public Sale(String name){
this.name = name;
}
/**
* 领取奖金
*/
synchronized public static void money(){
try {
System.out.println(Thread.currentThread().getName() + " 被领导表扬");
Thread.sleep(500);
System.out.println(Thread.currentThread().getName() + " 拿钱");
Thread.sleep(500);
System.out.println(Thread.currentThread().getName() + " 对公司表示感谢");
Thread.sleep(500);
System.out.println(Thread.currentThread().getName() + " 开开心心的拿钱走人");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Programmer{
private String name;
public Programmer(String name){
this.name = name;
}
/**
* 打开电脑
*/
synchronized public void computer(){
try {
System.out.println(this.name + " 接通电源");
Thread.sleep(500);
System.out.println(this.name + " 按开机按键");
Thread.sleep(500);
System.out.println(this.name + " 系统启动中");
Thread.sleep(500);
System.out.println(this.name + " 系统启动成功");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 编码
*/
synchronized public void coding(){
try {
System.out.println(this.name + " 双击Idea");
Thread.sleep(500);
System.out.println(this.name + " Idea启动完毕");
Thread.sleep(500);
System.out.println(this.name + " 开开心心的写代码");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 去卫生间
*/
public void wc(){
synchronized ("suibian") {
try {
System.out.println(this.name + " 打开卫生间门");
Thread.sleep(500);
System.out.println(this.name + " 开始排泄");
Thread.sleep(500);
System.out.println(this.name + " 冲水");
Thread.sleep(500);
System.out.println(this.name + " 离开卫生间");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/**
* 领取奖金
*/
public void money(){
synchronized (Programmer.class) {
try {
System.out.println(this.name + " 被领导表扬");
Thread.sleep(500);
System.out.println(this.name + " 拿钱");
Thread.sleep(500);
System.out.println(this.name + " 对公司表示感谢");
Thread.sleep(500);
System.out.println(this.name + " 开开心心的拿钱走人");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
class Manager{
private String name;
public Manager(String name){
this.name = name;
}
public String getName(){
return this.name;
}
/**
* 敬酒
*/
public void cheers(String mName,String eName){
try {
System.out.println(mName + " 来到 " + eName + " 面前");
Thread.sleep(500);
System.out.println(eName + " 拿起酒杯");
Thread.sleep(500);
System.out.println(mName + " 和 " + eName + " 干杯");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/**
* 打开电脑的工作线程
*/
class Working1 extends Thread{
private Programmer p;
public Working1(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.computer();
}
}
/**
* 编写代码的工作线程
*/
class Working2 extends Thread{
private Programmer p;
public Working2(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.coding();
}
}
/**
* 去卫生间的线程
*/
class WC extends Thread{
private Programmer p;
public WC(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.wc();
}
}
/**
* 程序员领取奖金
*/
class ProgrammerMoney extends Thread{
private Programmer p;
public ProgrammerMoney(Programmer p){
this.p = p;
}
@Override
public void run() {
this.p.money();
}
}
/**
* 销售部门领取奖金
*/
class SaleMoneyThread extends Thread{
private Sale p;
public SaleMoneyThread(Sale p){
this.p = p;
}
@Override
public void run() {
this.p.money();
}
}
/**
* 敬酒线程类
*/
class CheersThread extends Thread{
private Manager manager;
private String name;
public CheersThread(String name,Manager manager){
this.name = name;
this.manager = manager;
}
@Override
public void run() {
synchronized (this.manager) {
this.manager.cheers(this.manager.getName(), name);
}
}
}
public class TestSyncThread {
public static void main(String[] args) {
Manager manager = new Manager("张三丰");
new CheersThread("张三",manager).start();
new CheersThread("李四",manager).start();
}
}
死锁及解决方案
死锁的概念
“死锁”指的是：
多个线程各自占有一些共享资源，并且互相等待其他线程占有的资源才能进行，而导致两个或者多个线程都在等待对方释放资源，都停止执行的情形。
某一个同步块需要同时拥有“两个以上对象的锁”时，就可能会发生“死锁”的问题。比如，“化妆线程”需要同时拥有“镜子对象”、“口红对象”才能运行同步块。那么，实际运行时，“小丫的化妆线程”拥有了“镜子对象”，“大丫的化妆线程”拥有了“口红对象”，都在互相等待对方释放资源，才能化妆。这样，两个线程就形成了互相等待，无法继续运行的“死锁状态”。
死锁案例演示
/**
* 口红类
*/
class Lipstick{
}
/**
* 镜子类
*/
class Mirror{
}
/**
* 化妆线程类
*/
class Makeup extends Thread{
private int flag; //flag=0:拿着口红。flag!=0:拿着镜子
private String girlName;
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
public Makeup(int flag,String girlName){
this.flag = flag;
this.girlName = girlName;
}
@Override
public void run() {
this.doMakeup();
}
/**
* 开始化妆
*/
public void doMakeup(){
if(flag == 0){
synchronized (lipstick){
System.out.println(this.girlName+" 拿着口红");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (mirror){
System.out.println(this.girlName+" 拿着镜子");
}
}
}else{
synchronized (mirror){
System.out.println(this.girlName+" 拿着镜子");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lipstick){
System.out.println(this.girlName+" 拿着口红");
}
}
}
}
}
public class DeadLockThread {
public static void main(String[] args) {
new Makeup(0,"大丫").start();
new Makeup(1,"小丫").start();
}
}
死锁问题的解决
死锁是由于 “同步块需要同时持有多个对象锁造成”的，要解决这个问题，思路很简单，就是：同一个代码块，不要同时持有两个对象锁。
/**
* 口红类
*/
class Lipstick{
}
/**
* 镜子类
*/
class Mirror{
}
/**
* 化妆线程类
*/
class Makeup extends Thread{
private int flag; //flag=0:拿着口红。flag!=0:拿着镜子
private String girlName;
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
public void setFlag(int flag) {
this.flag = flag;
}
public void setGirlName(String girlName) {
this.girlName = girlName;
}
@Override
public void run() {
this.doMakeup();
}
/**
* 开始化妆
*/
public void doMakeup(){
if(flag == 0){
synchronized (lipstick){
System.out.println(this.girlName+" 拿着口红");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized (mirror){
System.out.println(this.girlName+" 拿着镜子");
}
}else{
synchronized (mirror){
System.out.println(this.girlName+" 拿着镜子");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized (lipstick){
System.out.println(this.girlName+" 拿着口红");
}
}
}
}
public class DeadLockThread {
public static void main(String[] args) {
Makeup makeup = new Makeup();
makeup.setFlag(0);
makeup.setGirlName("大丫");
Makeup makeup1 = new Makeup();
makeup1.setFlag(1);
makeup1.setGirlName("小丫");
makeup.start();
makeup1.start();
}
}