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什么是线程同步
同步问题的提出
现实生活中#xff0c;我们会遇到“同一个资源#xff0c;多个人都想使用”的问题。 比如#xff1a;教室里#xff0c;只有一台电脑#xff0c;多个人都想使用。天然的解决办法就是#xff0c;在电脑旁边#xff0c;大家排队…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*/Overridepublic 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*/Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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;}Overridepublic 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; //flag0: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;}Overridepublic 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; //flag0: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;}Overridepublic 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();}
}
