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Java通俗易懂30天 第19天:多线程(下)_java 多线程 通俗易懂
作者：寸_铁 | 2024-08-10 01:26:29
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java 多线程 通俗易懂
文章目录
全部源码：https://github.com/name365/JavaSE-30Day
第8章 多线程
线程的生命周期
- JDK中用Thread.State类定义了线程的几种状态
要想实现多线程，必须在主线程中创建新的线程对象。Java语言使用Thread类及其子类的对象来表示线程，在它的一个完整的生命周期中通常要经历如下的五种状态：
- 新建：当一个Thread类或其子类的对象被声明并创建时，新生的线程对象处于新建状态
- 就绪：处于新建状态的线程被start()后，将进入线程队列等待CPU时间片，此时它已具备了运行的条件，只是没分配到CPU资源
- 运行：当就绪的线程被调度并获得CPU资源时,便进入运行状态，run()方法定义了线程的操作和功能
- 阻塞：在某种特殊情况下，被人为挂起或执行输入输出操作时，让出CPU并临时中止自己的执行，进入阻塞状态
- 死亡：线程完成了它的全部工作或线程被提前强制性地中止或出现异常导致结束
- 线程的生命周期
线程的同步
问题的提出
多个线程执行的不确定性引起执行结果的不稳定
多个线程对账本的共享，会造成操作的不完整性，会破坏数据。
- 例题——模拟火车站售票程序，开启三个窗口售票。
class Windows1 implements Runnable{
private int ticket = 100;
@Override
public void run() {
while(true){
if(ticket > 0){
System.out.println(Thread.currentThread().getName() + ":卖票，票号为: " + ticket);
ticket--;
}else{
break;
}
}
}
}
public class WindowsTest1 {
public static void main(String[] args) {
Windows1 w = new Windows1();
Thread t1 = new Thread(w);
Thread t2 = new Thread(w);
Thread t3 = new Thread(w);
t1.setName("窗口1");
t2.setName("窗口2");
t3.setName("窗口3");
t1.start();
t2.start();
t3.start();
}
}
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- 理想状态
- 极端状态
同步代码块处理实现Runnable的线程安全问题
class Windows1 implements Runnable{
private int ticket = 100;
@Override
public void run() {
while(true){
synchronized (this) {
if (ticket > 0) {
try{
Thread.sleep(100);
}catch (InterruptedException e){
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ":卖票，票号为: " + ticket);
ticket--;
} else {
break;
}
}
}
}
}
public class WindowsTest1 {
public static void main(String[] args) {
Windows1 w = new Windows1();
Thread t1 = new Thread(w);
Thread t2 = new Thread(w);
Thread t3 = new Thread(w);
t1.setName("窗口1");
t2.setName("窗口2");
t3.setName("窗口3");
t1.start();
t2.start();
t3.start();
}
}
class Dog{
}
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- 分析同步原理
同步代码块处理继承Thread类的线程安全问题
class Windows extends Thread{
private static int ticket = 100;
private static Object obj = new Object();
@Override
public void run() {
while(true){
synchronized (Windows.class){
if (ticket > 0) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(getName() + ":卖票，票号为: " + ticket);
ticket--;
} else {
break;
}
}
}
}
}
public class WindowsTest2 {
public static void main(String[] args) {
Windows t1 = new Windows();
Windows t2 = new Windows();
Windows t3 = new Windows();
t1.setName("窗口1");
t2.setName("窗口2");
t3.setName("窗口3");
t1.start();
t2.start();
t3.start();
}
}
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同步方法处理实现Runnable的线程安全问题
class Windows3 implements Runnable {
private int ticket = 100;
@Override
public void run() {
while (true) {
show();
}
}
public synchronized void show() {
if (ticket > 0) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ":卖票，票号为: " + ticket);
ticket--;
}
}
}
public class WindowsTest3 {
public static void main(String[] args) {
Windows3 w3 = new Windows3();
Thread t1 = new Thread(w3);
Thread t2 = new Thread(w3);
Thread t3 = new Thread(w3);
t1.setName("窗口1");
t2.setName("窗口2");
t3.setName("窗口3");
t1.start();
t2.start();
t3.start();
}
}
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同步方法处理继承Thread类的线程安全问题
class Windows4 extends Thread {
private static int ticket = 100;
@Override
public void run() {
while (true) {
show();
}
}
private static synchronized void show(){
if (ticket > 0) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "：卖票，票号为：" + ticket);
ticket--;
}
}
}
public class WindowsTest4 {
public static void main(String[] args) {
Windows4 t1 = new Windows4();
Windows4 t2 = new Windows4();
Windows4 t3 = new Windows4();
t1.setName("窗口1");
t2.setName("窗口2");
t3.setName("窗口3");
t1.start();
t2.start();
t3.start();
}
}
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线程安全的单例模式之懒汉式
public class BankTest {
}
class Bank{
private Bank(){}
private static Bank instance = null;
public static Bank getInstance(){
if(instance == null) {
synchronized (Bank.class) {
if (instance == null) {
instance = new Bank();
}
}
}
return instance;
}
}
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死锁的问题
- 例1
public class ThreadTest {
public static void main(String[] args) {
StringBuffer s1 = new StringBuffer();
StringBuffer s2 = new StringBuffer();
new Thread(){
@Override
public void run() {
synchronized (s1){
s1.append("a");
s2.append("1");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (s2){
s1.append("b");
s2.append("2");
System.out.println(s1);
System.out.println(s2);
}
}
}
}.start();
new Thread(new Runnable() {
@Override
public void run() {
synchronized (s2){
s1.append("c");
s2.append("3");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (s1){
s1.append("d");
s2.append("4");
System.out.println(s1);
System.out.println(s2);
}
}
}
}).start();
}
}
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- 例2
class A {
public synchronized void foo(B b) {
System.out.println("当前线程名: " + Thread.currentThread().getName()
+ " 进入了A实例的foo方法");
try {
Thread.sleep(200);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.println("当前线程名: " + Thread.currentThread().getName()
+ " 企图调用B实例的last方法");
b.last();
}
public synchronized void last() {
System.out.println("进入了A类的last方法内部");
}
}
class B {
public synchronized void bar(A a) {
System.out.println("当前线程名: " + Thread.currentThread().getName()
+ " 进入了B实例的bar方法");
try {
Thread.sleep(200);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.println("当前线程名: " + Thread.currentThread().getName()
+ " 企图调用A实例的last方法");
a.last();
}
public synchronized void last() {
System.out.println("进入了B类的last方法内部");
}
}
public class DeadLock implements Runnable {
A a = new A();
B b = new B();
public void init() {
Thread.currentThread().setName("主线程");
a.foo(b);
System.out.println("进入了主线程之后");
}
public void run() {
Thread.currentThread().setName("副线程");
b.bar(a);
System.out.println("进入了副线程之后");
}
public static void main(String[] args) {
DeadLock dl = new DeadLock();
new Thread(dl).start();
dl.init();
}
}
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Lock锁方式解决线程安全问题
- 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 类实现了Lock ，它拥有与synchronized 相同的并发性和内存语义，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。
- 从JDK 5.0开始，Java提供了更强大的线程同步机制——通过显式定义同步锁对象来实现同步。同步锁使用Lock对象充当。
import java.util.concurrent.locks.ReentrantLock;
class Windows implements Runnable{
private int ticket = 100;
private ReentrantLock lock = new ReentrantLock();
@Override
public void run() {
while(true){
try{
lock.lock();
if(ticket > 0){
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ":售票，票号为: " + ticket);
ticket --;
}else{
break;
}
}finally {
lock.unlock();
}
}
}
}
public class LockTest {
public static void main(String[] args) {
Windows w = new Windows();
Thread t1 = new Thread(w);
Thread t2 = new Thread(w);
Thread t3 = new Thread(w);
t1.setName("窗口1");
t2.setName("窗口2");
t3.setName("窗口3");
t1.start();
t2.start();
t3.start();
}
}
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- 练习
class Account{
private double balance;
public Account(double balance){
this.balance = balance;
}
public synchronized void deposit(double amt){
if(amt > 0){
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
balance += amt;
System.out.println(Thread.currentThread().getName() + ":" + "存钱成功，当前余额:" + balance);
}
}
}
class Customer extends Thread{
private Account acct;
public Customer(Account acct){
this.acct = acct;
}
@Override
public void run() {
for(int i = 0;i < 3;i++){
acct.deposit(1000);
}
}
}
public class AccountTest {
public static void main(String[] args) {
Account acct = new Account(0);
Customer c1 = new Customer(acct);
Customer c2 = new Customer(acct);
c1.setName("甲");
c2.setName("乙");
c1.start();
c2.start();
}
}
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线程的通信
class Number implements Runnable{
private int number = 1;
public Object obj = new Object();
@Override
public void run() {
while (true){
synchronized (obj) {
obj.notify();
if(number 100){
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ":" + number);
number++;
try {
obj.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}else{
break;
}
}
}
}
}
public class CommunicationTest {
public static void main(String[] args) {
Number number = new Number();
Thread t1 = new Thread(number);
Thread t2 = new Thread(number);
t1.setName("线程1");
t2.setName("线程2");
t1.start();
t2.start();
}
}
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sleep()和wait()的异同
经典例题：生产者/消费者问题
class Clerk{
private int productCount = 0;
public synchronized void produceProduct() {
if(productCount < 20){
productCount++;
System.out.println(Thread.currentThread().getName() + ": 开始生产第" + productCount + "个产品");
notify();
}else{
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public synchronized void consumeProduct() {
if(productCount > 0){
System.out.println(Thread.currentThread().getName() + ":开始消费第" + productCount + "个产品");
productCount--;
notify();
}else{
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
class Producer extends Thread{
private Clerk clerk;
public Producer(Clerk clerk){
this.clerk = clerk;
}
@Override
public void run() {
System.out.println(getName() + ": 开始生产产品......");
while(true){
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
clerk.produceProduct();
}
}
}
class Consumer extends Thread{
private Clerk clerk;
public Consumer(Clerk clerk){
this.clerk = clerk;
}
@Override
public void run() {
System.out.println(getName() + ": 开始消费产品......");
while(true){
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
clerk.consumeProduct();
}
}
}
public class ProductTest {
public static void main(String[] args) {
Clerk clerk = new Clerk();
Producer p1 = new Producer(clerk);
p1.setName("生产者1");
Consumer c1 = new Consumer(clerk);
c1.setName("消费者1");
Consumer c2 = new Consumer(clerk);
c2.setName("消费者2");
p1.start();
c1.start();
c2.start();
}
}
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JDK5.0新增线程创建方式
创建多线程的方式三：实现Callable接口
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
class NumThread implements Callable{
@Override
public Object call() throws Exception {
int sum = 0;
for(int i = 1;i 100;i++){
if(i % 2 == 0){
System.out.println(i);
sum += i;
}
}
return sum;
}
}
public class ThreadNew {
public static void main(String[] args) {
NumThread numThread = new NumThread();
FutureTask futureTask = new FutureTask(numThread);
new Thread(futureTask).start();
try {
Object sum = futureTask.get();
System.out.println("总和为:" + sum);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
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- Future接口
- 可以对具体Runnable、Callable任务的执行结果进行取消、查询是否完成、获取结果等。
- FutrueTask是Futrue接口的唯一的实现类
- FutureTask 同时实现了Runnable, Future接口。它既可以作为Runnable被线程执行，又可以作为Future得到Callable的返回值
使用线程池的好处
背景：经常创建和销毁、使用量特别大的资源，比如并发情况下的线程，对性能影响很大。
思路：提前创建好多个线程，放入线程池中，使用时直接获取，使用完放回池中。可以避免频繁创建销毁、实现重复利用。类似生活中的公共交通工具。
好处：
- 提高响应速度（减少了创建新线程的时间）
- 降低资源消耗（重复利用线程池中线程，不需要每次都创建）
- 便于线程管理
- corePoolSize：核心池的大小
- maximumPoolSize：最大线程数
- keepAliveTime：线程没有任务时最多保持多长时间后会终止
- …
创建多线程的方式四：使用线程池
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
class NumberThread implements Runnable{
@Override
public void run() {
for(int i = 0;i 100;i++){
if(i % 2 == 0){
System.out.println(Thread.currentThread().getName() + ":" + i);
}
}
}
}
class NumberThread1 implements Runnable{
@Override
public void run() {
for(int i = 0;i 100;i++){
if(i % 2 != 0){
System.out.println(Thread.currentThread().getName() + ":" + i);
}
}
}
}
public class ThreadPool {
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(10);
ThreadPoolExecutor service1 = (ThreadPoolExecutor) service;
service.execute(new NumberThread());
service.execute(new NumberThread1());
service.shutdown();
}
}
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- 线程池相关API
- JDK 5.0起提供了线程池相关API： ExecutorService和 Executors
- ExecutorService：真正的线程池接口。常见子类ThreadPoolExecutor
- void execute(Runnable command) ：执行任务/命令，没有返回值，一般用来执行Runnable
- Future submit(Callable task)：执行任务，有返回值，一般又来执行Callable
- void shutdown() ：关闭连接池
- Executors：工具类、线程池的工厂类，用于创建并返回不同类型的线程池
- Executors.newCachedThreadPool()：创建一个可根据需要创建新线程的线程池
- Executors.newFixedThreadPool(n); 创建一个可重用固定线程数的线程池
- Executors.newSingleThreadExecutor() ：创建一个只有一个线程的线程池
- Executors.newScheduledThreadPool(n)：创建一个线程池，它可安排在给定延迟后运行命令或者定期地执行。
整个Java全栈系列都是笔者自己敲的笔记。写作不易，如果可以，点个赞呗！✌
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