一、关于Java多线程中的一些概念
1.1 线程基本概念
从JDK1.5开始，Java提供了3中方式来创建、启动多线程：
方式一（不推荐）、通过继承Thread类来创建线程类，重写run()方法作为线程执行体；
方式二、实现Runnable接口来创建线程类，重写run()方法作为线程执行体；
方式三、实现Callable接口来创建线程类，重写run()方法作为线程执行体；
不同的是，其中方式一的效果最差，是因为
1、线程类继承了Thread类，无法再继承其他父类；
2、因为每条线程都是一个Thread子类的实例，因此多个线程之间共享数据比较麻烦。
二、一些关于synchronized关键字的简单Demo
2.1、多个线程单个锁
当多个线程访问myThread的run方法时，以排队的方式进行处理（这里排对是按照CPU分配的先后顺序而定的），一个线程想要执行synchronized修饰的方法里的代码首先会去尝试获得锁，如果拿到锁，执行synchronized代码体内容；
如果拿不到锁，这个线程就会不断的尝试获得这把锁，直到拿到为止，而且是多个线程同时去竞争这把锁。（也就是会有锁竞争的问题）
package com.sun.multithread.sync;public class MyThread extends Thread { private int count = 5; // synchronized加锁
public synchronized void run() {
count--;
System.out.println(this.currentThread().getName() + " count = " + count);
} public static void main(String[] args) { MyThread myThread = new MyThread();
Thread t1 = new Thread(myThread, "t1");
Thread t2 = new Thread(myThread, "t2");
Thread t3 = new Thread(myThread, "t3");
Thread t4 = new Thread(myThread, "t4");
Thread t5 = new Thread(myThread, "t5"); t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
}
}
2.2、多个线程多把锁
关键字synchronized取得的锁都是对象锁，而不是把一段代码（方法）当成锁，所以代码中哪个线程先执行synchronized关键字的方法，哪个线程就持有该方法所属对象的锁。但是在静态(static)方法上加synchronized关键字，表示锁定class类，类级别的锁
package com.sun.multithread.sync;public class MultiThread { private static int num = 0; /**
* 在静态(static)方法上加synchronized关键字，表示锁定class类，类级别的锁
*
* 关键字synchronized取得的锁都是对象锁，而不是把一段代码（方法）当成锁，
* 所以代码中哪个线程先执行synchronized关键字的方法，哪个线程就持有该方法所属对象的锁
*
* @param tag 参数
*/
public static synchronized void printNum(String tag){
try {
if(tag.equals("a")){
num = 100;
System.out.println("tag a, set num over!");
Thread.sleep(1000);
} else {
num = 200;
System.out.println("tag b, set num over!");
}
System.out.println("tag " + tag + ", num = " + num);
} catch (InterruptedException e) {
e.printStackTrace();
}
} public static void main(String[] args) {
// 两个不同的对象
final MultiThread m1 = new MultiThread();
final MultiThread m2 = new MultiThread(); Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
m1.printNum("a");
}
}); Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
m1.printNum("b");
}
}); t1.start();
t2.start();
}
}
2.3 银行取钱的多线程例子
1、创建一个银行账户，并在里面写好取钱的方法，其中使用synchronized关键字修饰多线程操作的方法
package com.sun.multithread.sync.bankdemo;/**
* 银行账户类
*
* @author ietree
*/
public class Account { /**
* 账号
*/
private String accountNo; /**
* 余额
*/
private double balance; public Account() {
} /**
* 带参构造函数
*
* @param accountNo 账户
* @param balance 余额
*/
public Account(String accountNo, double balance) {
this.accountNo = accountNo;
this.balance = balance;
} // 访问该账户的余额，使用synchronized修饰符将它变成同步方法
public synchronized double getBalance() {
return balance;
} /**
* 取钱的方法
*
* @param drawAmount 取钱金额
*/
public synchronized void draw(double drawAmount) { // 如果余额大于等于用户取的钱，则取款成功
if (balance >= drawAmount) {
// 取款成功
System.out.println(Thread.currentThread().getName() + "取钱成功！用户取出" + drawAmount + "元"); // 修改余额
balance -= drawAmount;
System.out.println("\t余额为： " + balance); } else { System.out.println(Thread.currentThread().getName() + "取钱失败！您的余额不足"); } } public String getAccountNo() {
return accountNo;
} public void setAccountNo(String accountNo) {
this.accountNo = accountNo;
} // 重写hashCode()方法
public int hashCode() {
return accountNo.hashCode();
} // 重写equals()方法
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj != null && obj.getClass() == Account.class) {
Account target = (Account) obj;
return target.accountNo.equals(accountNo);
}
return false;
}}
2、创建多个线程同时操作一个账户
package com.sun.multithread.sync.bankdemo;class DrawThread extends Thread { /**
* 模拟用户账户
*/
private Account account; /**
* 当前取钱线程所希望取的钱数
*/
private double drawAmount; public DrawThread(String name, Account account, double drawAmount){
super(name);
this.account = account;
this.drawAmount = drawAmount;
} // 当多条线程修改同一个共享数据时，将涉及数据安全问题
public void run(){
account.draw(drawAmount);
}
}public class DrawTest { public static void main(String[] args) {
// 创建一个账户
Account acct = new Account("1234567", 1000); // 模拟两个线程对同一账户取钱
new DrawThread("路人甲", acct, 800).start();
new DrawThread("路人乙", acct, 800).start();
}}
2.4 业务整体需要使用完整的synchronized，保持业务的原子性
package com.ietree.multithread.sync;/**
* 业务整体需要使用完整的synchronized，保持业务的原子性
*
* @author ietree
*/
public class DirtyRead { private String username = "Jack";
private String password = "123"; public synchronized void setValue(String username, String password) {
this.username = username; try {
// 睡眠2秒
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
} this.password = password; System.out.println("setValue最终结果：username = " + username + ", password = " + password);
} // synchronized
public void getValue() {
System.out.println("getValue方法得到：username = " + this.username + ", password = " + this.password);
} public static void main(String[] args) throws Exception { final DirtyRead dr = new DirtyRead();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
dr.setValue("Dylan", "456");
}
});
t1.start();
Thread.sleep(1000); dr.getValue();
}}
程序输出：
getValue方法得到：username = Dylan, password = 123
setValue最终结果：username = Dylan, password = 456
这里出现了脏读现象，应该要保持业务的原子性，修改如下：
package com.ietree.multithread.sync;/**
* 业务整体需要使用完整的synchronized，保持业务的原子性
*
* @author ietree
*/
public class DirtyRead { private String username = "Jack";
private String password = "123"; public synchronized void setValue(String username, String password) {
this.username = username; try {
// 睡眠2秒
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
} this.password = password; System.out.println("setValue最终结果：username = " + username + ", password = " + password);
} // synchronized
public synchronized void getValue() {
System.out.println("getValue方法得到：username = " + this.username + ", password = " + this.password);
} public static void main(String[] args) throws Exception { final DirtyRead dr = new DirtyRead();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
dr.setValue("Dylan", "456");
}
});
t1.start();
Thread.sleep(1000); dr.getValue();
}}
程序输出：
setValue最终结果：username = Dylan, password = 456
getValue方法得到：username = Dylan, password = 456
当在set和get方法上同时使用了synchronized能确保业务原子性，不会出现脏读现象。
2.5 synchronized的重入
demo1：
package com.ietree.multithread.sync;public class SyncDemo1 { public synchronized void method1(){
System.out.println("method1..");
method2();
}
public synchronized void method2(){
System.out.println("method2..");
method3();
}
public synchronized void method3(){
System.out.println("method3..");
} public static void main(String[] args) {
final SyncDemo1 sd = new SyncDemo1();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
sd.method1();
}
});
t1.start();
}}
demo2：
package com.ietree.multithread.sync;public class SyncDemo2 { static class Parent {
public int i = 10; public synchronized void operationSup() {
try {
i--;
System.out.println("Main print i = " + i);
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} static class Sub extends Parent {
public synchronized void operationSub() {
try {
while (i > 0) {
i--;
System.out.println("Sub print i = " + i);
Thread.sleep(100);
this.operationSup();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
Sub sub = new Sub();
sub.operationSub();
}
}); t1.start();
}
}
2.6 synchronized的Exception
package com.ietree.multithread.sync;public class SyncException { private int i = 0; public synchronized void operation() {
while (true) {
try {
i++;
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + " , i = " + i);
if (i == 20) {
throw new RuntimeException();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { final SyncException se = new SyncException();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
se.operation();
}
}, "t1");
t1.start();
}
}
2.7 锁对象的改变问题
demo1:
package com.ietree.multithread.sync;public class ChangeLock { private String lock = "lock"; private void method() {
synchronized (lock) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + "开始");
lock = "change lock";
Thread.sleep(2000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + "结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { final ChangeLock changeLock = new ChangeLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
changeLock.method();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
changeLock.method();
}
}, "t2");
t1.start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
程序输出：
当前线程 : t1开始
当前线程 : t2开始
当前线程 : t1结束
当前线程 : t2结束
demo2:
package com.ietree.multithread.sync;public class ChangeLock { private String lock = "lock"; private void method() {
synchronized (lock) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + "开始");
// lock = "change lock";
Thread.sleep(2000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + "结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { final ChangeLock changeLock = new ChangeLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
changeLock.method();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
changeLock.method();
}
}, "t2");
t1.start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
程序输出：
当前线程 : t1开始
当前线程 : t1结束
当前线程 : t2开始
当前线程 : t2结束
两个程序相比差异在于是否含有 lock = “change lock”;这个改变了锁对象，所以输出有差异。
2.8 死锁问题，在设计程序时就应该避免双方相互持有对方的锁的情况
package com.ietree.multithread.sync;public class DeadLock implements Runnable { private String tag;
private static Object lock1 = new Object();
private static Object lock2 = new Object(); public void setTag(String tag) {
this.tag = tag;
} @Override
public void run() {
if (tag.equals("a")) {
synchronized (lock1) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock1执行");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock2) {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock2执行");
}
}
}
if (tag.equals("b")) {
synchronized (lock2) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock2执行");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock1) {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock1执行");
}
}
}
} public static void main(String[] args) { DeadLock d1 = new DeadLock();
d1.setTag("a");
DeadLock d2 = new DeadLock();
d2.setTag("b"); Thread t1 = new Thread(d1, "t1");
Thread t2 = new Thread(d2, "t2"); t1.start();
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
程序输出：
当前线程 : t1 进入lock1执行
当前线程 : t2 进入lock2执行
程序一直等待获取锁的状态，造成死锁问题。
2.9 同一对象属性的修改不会影响锁的情况
package com.ietree.multithread.sync;public class ModifyLock {
private String name;
private int age; public String getName() {
return name;
} public void setName(String name) {
this.name = name;
} public int getAge() {
return age;
} public void setAge(int age) {
this.age = age;
} public synchronized void changeAttributte(String name, int age) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 开始");
this.setName(name);
this.setAge(age); System.out.println("当前线程 : " + Thread.currentThread().getName() + " 修改对象内容为： " + this.getName() + ", "
+ this.getAge()); Thread.sleep(2000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
} public static void main(String[] args) { final ModifyLock modifyLock = new ModifyLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
modifyLock.changeAttributte("Jack", 18);
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
modifyLock.changeAttributte("Rose", 20);
}
}, "t2"); t1.start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
程序输出：
当前线程 : t1 开始
当前线程 : t1 修改对象内容为： Jack, 18
当前线程 : t1 结束
当前线程 : t2 开始
当前线程 : t2 修改对象内容为： Rose, 20
当前线程 : t2 结束
2.10 使用synchronized代码块加锁,比较灵活
package com.ietree.multithread.sync;public class ObjectLock {
public void method1() {
synchronized (this) { // 对象锁
try {
System.out.println("do method1..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public void method2() { // 类锁
synchronized (ObjectLock.class) {
try {
System.out.println("do method2..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} private Object lock = new Object(); public void method3() { // 任何对象锁
synchronized (lock) {
try {
System.out.println("do method3..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { final ObjectLock objLock = new ObjectLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method1();
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method2();
}
});
Thread t3 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method3();
}
}); t1.start();
t2.start();
t3.start(); }
}
程序输出：
do method2..
do method3..
do method1..
以上语句是同时输出的，因为他们拿到的锁都是不同的锁，所以互不影响。
2.11 使用synchronized代码块减小锁的粒度，提高性能
package com.ietree.multithread.sync;public class Optimize { public void doLongTimeTask() {
try {
System.out.println("当前线程开始：" + Thread.currentThread().getName() + ", 正在执行一个较长时间的业务操作，其内容不需要同步");
Thread.sleep(2000); synchronized (this) {
System.out.println("当前线程：" + Thread.currentThread().getName() + ", 执行同步代码块，对其同步变量进行操作");
Thread.sleep(1000);
}
System.out.println("当前线程结束：" + Thread.currentThread().getName() + ", 执行完毕"); } catch (InterruptedException e) {
e.printStackTrace();
}
} public static void main(String[] args) {
final Optimize otz = new Optimize();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
otz.doLongTimeTask();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
otz.doLongTimeTask();
}
}, "t2");
t1.start();
t2.start(); }
}
程序输出：
当前线程开始：t2, 正在执行一个较长时间的业务操作，其内容不需要同步
当前线程开始：t1, 正在执行一个较长时间的业务操作，其内容不需要同步
当前线程：t1, 执行同步代码块，对其同步变量进行操作
当前线程结束：t1, 执行完毕
当前线程：t2, 执行同步代码块，对其同步变量进行操作
当前线程结束：t2, 执行完毕
2.12 避免使用字符串常量锁
package com.ietree.multithread.sync;public class StringLock { public void method() {
// new String("字符串常量")
synchronized ("字符串常量") {
try {
while (true) {
System.out.println("当前线程 : " + Thread.currentThread().getName() + "开始");
Thread.sleep(1000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + "结束");
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) {
final StringLock stringLock = new StringLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
stringLock.method();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
stringLock.method();
}
}, "t2"); t1.start();
t2.start();
}
}
程序输出：
当前线程 : t1开始
当前线程 : t1结束
当前线程 : t1开始
当前线程 : t1结束
当前线程 : t1开始
当前线程 : t1结束
当前线程 : t1开始
......
