java-多线程（上）

###24.01_多线程(多线程的引入)(了解)
* 1.什么是线程
    * 线程是程序执行的一条路径, 一个进程中可以包含多条线程
    * 多线程并发执行可以提高程序的效率, 可以同时完成多项工作
* 2.多线程的应用场景
    * 红蜘蛛同时共享屏幕给多个电脑
    * 迅雷开启多条线程一起下载
    * QQ同时和多个人一起视频
    * 服务器同时处理多个客户端请求
    
###24.02_多线程(多线程并行和并发的区别)(了解)
* 并行就是两个任务同时运行，就是甲任务进行的同时，乙任务也在进行。(需要多核CPU)
* 并发是指两个任务都请求运行，而处理器只能按受一个任务，就把这两个任务安排轮流进行，由于时间间隔较短，使人感觉两个任务都在运行。
* 比如我跟两个网友聊天，左手操作一个电脑跟甲聊，同时右手用另一台电脑跟乙聊天，这就叫并行。
* 如果用一台电脑我先给甲发个消息，然后立刻再给乙发消息，然后再跟甲聊，再跟乙聊。这就叫并发。

###24.03_多线程(Java程序运行原理和JVM的启动是多线程的吗)(了解)
* A:Java程序运行原理
    * Java命令会启动java虚拟机，启动JVM，等于启动了一个应用程序，也就是启动了一个进程。该进程会自动启动一个 “主线程” ，然后主线程去调用某个类的 main 方法。
    
* B:JVM的启动是多线程的吗
    * JVM启动至少启动了垃圾回收线程和主线程，所以是多线程的。

###24.04_多线程(多线程程序实现的方式1)(掌握)
* 1.继承Thread
    * 定义类继承Thread
    * 重写run方法
    * 把新线程要做的事写在run方法中
    * 创建线程对象
    * 开启新线程, 内部会自动执行run方法
    * 
        
            public class Demo2_Thread {
        
                /**
                 * @param args
                 */
                public static void main(String[] args) {
                    MyThread mt = new MyThread();                            //4,创建自定义类的对象
                    mt.start();                                                //5,开启线程
                    
                    for(int i = 0; i < 3000; i++) {
                        System.out.println("bb");
                    }
                }
            
            }
            class MyThread extends Thread {                                    //1,定义类继承Thread
                public void run() {                                            //2,重写run方法
                    for(int i = 0; i < 3000; i++) {                            //3,将要执行的代码,写在run方法中
                        System.out.println("aaaaaaaaaaaaaaaaaaaaaaaaaaaa");
                    }
                }
            }

###24.05_多线程(多线程程序实现的方式2)(掌握)
* 2.实现Runnable
    * 定义类实现Runnable接口
    * 实现run方法
    * 把新线程要做的事写在run方法中
    * 创建自定义的Runnable的子类对象
    * 创建Thread对象, 传入Runnable
    * 调用start()开启新线程, 内部会自动调用Runnable的run()方法

public class Demo3_Runnable {
                /**
                 * @param args
                 */
                public static void main(String[] args) {
                    MyRunnable mr = new MyRunnable();                        //4,创建自定义类对象
                    //Runnable target = new MyRunnable();
                    Thread t = new Thread(mr);                                //5,将其当作参数传递给Thread的构造函数
                    t.start();                                                //6,开启线程
                    
                    for(int i = 0; i < 3000; i++) {
                        System.out.println("bb");
                    }
                }
            }
            
            class MyRunnable implements Runnable {                            //1,自定义类实现Runnable接口
                @Override
                public void run() {                                            //2,重写run方法
                    for(int i = 0; i < 3000; i++) {                            //3,将要执行的代码,写在run方法中
                        System.out.println("aaaaaaaaaaaaaaaaaaaaaaaaaaaa");
                    }
                }
                
            }

###24.06_多线程(实现Runnable的原理)(了解)
* 查看源码
    * 1,看Thread类的构造函数,传递了Runnable接口的引用 
    * 2,通过init()方法找到传递的target给成员变量的target赋值
    * 3,查看run方法,发现run方法中有判断,如果target不为null就会调用Runnable接口子类对象的run方法

###24.07_多线程(两种方式的区别)(掌握)
* 查看源码的区别:
    * a.继承Thread : 由于子类重写了Thread类的run(), 当调用start()时, 直接找子类的run()方法
    * b.实现Runnable : 构造函数中传入了Runnable的引用, 成员变量记住了它, start()调用run()方法时内部判断成员变量Runnable的引用是否为空, 不为空编译时看的是Runnable的run(),运行时执行的是子类的run()方法
    
* 继承Thread
    * 好处是:可以直接使用Thread类中的方法,代码简单
    * 弊端是:如果已经有了父类,就不能用这种方法
* 实现Runnable接口
    * 好处是:即使自己定义的线程类有了父类也没关系,因为有了父类也可以实现接口,而且接口是可以多实现的
    * 弊端是:不能直接使用Thread中的方法需要先获取到线程对象后,才能得到Thread的方法,代码复杂
###24.08_多线程(匿名内部类实现线程的两种方式)(掌握)
* 继承Thread类
         
        new Thread() {                                                    //1,new 类(){}继承这个类
            public void run() {                                            //2,重写run方法
                for(int i = 0; i < 3000; i++) {                            //3,将要执行的代码,写在run方法中
                    System.out.println("aaaaaaaaaaaaaaaaaaaaaaaaaaaa");
                }
            }
        }.start();
* 实现Runnable接口
            
        new Thread(new Runnable(){                                        //1,new 接口(){}实现这个接口
            public void run() {                                            //2,重写run方法
                for(int i = 0; i < 3000; i++) {                            //3,将要执行的代码,写在run方法中
                    System.out.println("bb");
                }
            }
        }).start();

###24.09_多线程(获取名字和设置名字)(掌握)
* 1.获取名字
    * 通过getName()方法获取线程对象的名字
* 2.设置名字
    * 通过构造函数可以传入String类型的名字
    * 
            new Thread("xxx") {
                public void run() {
                    for(int i = 0; i < 1000; i++) {
                        System.out.println(this.getName() + "....aaaaaaaaaaaaaaaaaaaaaaa");
                    }
                }
            }.start();
            
            new Thread("yyy") {
                public void run() {
                    for(int i = 0; i < 1000; i++) {
                        System.out.println(this.getName() + "....bb");
                    }
                }
            }.start(); 
    * 通过setName(String)方法可以设置线程对象的名字
    * 
            Thread t1 = new Thread() {
                public void run() {
                    for(int i = 0; i < 1000; i++) {
                        System.out.println(this.getName() + "....aaaaaaaaaaaaaaaaaaaaaaa");
                    }
                }
            };
            
            Thread t2 = new Thread() {
                public void run() {
                    for(int i = 0; i < 1000; i++) {
                        System.out.println(this.getName() + "....bb");
                    }
                }
            };
            t1.setName("芙蓉姐姐");
            t2.setName("凤姐");
            
            t1.start();
            t2.start();

###24.10_多线程(获取当前线程的对象)(掌握)
* Thread.currentThread(), 主线程也可以获取
    * 
            new Thread(new Runnable() {
                public void run() {
                    for(int i = 0; i < 1000; i++) {
                        System.out.println(Thread.currentThread().getName() + "...aaaaaaaaaaaaaaaaaaaaa");
                    }
                }
            }).start();
            
            new Thread(new Runnable() {
                public void run() {
                    for(int i = 0; i < 1000; i++) {
                        System.out.println(Thread.currentThread().getName() + "...bb");
                    }
                }
            }).start();
            Thread.currentThread().setName("我是主线程");                    //获取主函数线程的引用,并改名字
            System.out.println(Thread.currentThread().getName());        //获取主函数线程的引用,并获取名字
###24.11_多线程(休眠线程)(掌握)
* Thread.sleep(毫秒,纳秒), 控制当前线程休眠若干毫秒1秒= 1000毫秒 1秒 = 1000 * 1000 * 1000纳秒 1000000000

new Thread() {
                public void run() {
                    for(int i = 0; i < 10; i++) {
                        System.out.println(getName() + "...aaaaaaaaaaaaaaaaaaaaaa");
                        try {
                            Thread.sleep(10);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }.start();
            
            new Thread() {
                public void run() {
                    for(int i = 0; i < 10; i++) {
                        System.out.println(getName() + "...bb");
                        try {
                            Thread.sleep(10);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }.start();
###24.12_多线程(守护线程)(掌握)
* setDaemon(), 设置一个线程为守护线程, 该线程不会单独执行, 当其他非守护线程都执行结束后, 自动退出
    * 
            Thread t1 = new Thread() {
                public void run() {
                    for(int i = 0; i < 50; i++) {
                        System.out.println(getName() + "...aaaaaaaaaaaaaaaaaaaaaa");
                        try {
                            Thread.sleep(10);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            };
            
            Thread t2 = new Thread() {
                public void run() {
                    for(int i = 0; i < 5; i++) {
                        System.out.println(getName() + "...bb");
                        try {
                            Thread.sleep(10);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            };
            
            t1.setDaemon(true);                        //将t1设置为守护线程
            
            t1.start();
            t2.start();
###24.13_多线程(加入线程)(掌握)
* join(), 当前线程暂停, 等待指定的线程执行结束后, 当前线程再继续
* join(int), 可以等待指定的毫秒之后继续
    * 
            final Thread t1 = new Thread() {
                public void run() {
                    for(int i = 0; i < 50; i++) {
                        System.out.println(getName() + "...aaaaaaaaaaaaaaaaaaaaaa");
                        try {
                            Thread.sleep(10);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            };
            
            Thread t2 = new Thread() {
                public void run() {
                    for(int i = 0; i < 50; i++) {
                        if(i == 2) {
                            try {
                                //t1.join();                        //插队,加入
                                t1.join(30);                        //加入,有固定的时间,过了固定时间,继续交替执行
                                Thread.sleep(10);
                            } catch (InterruptedException e) {
                                
                                e.printStackTrace();
                            }
                        }
                        System.out.println(getName() + "...bb");
                    
                    }
                }
            };
            
            t1.start();
            t2.start();
###24.14_多线程(礼让线程)(了解)
* yield让出cpu

###24.15_多线程(设置线程的优先级)(了解)
* setPriority()设置线程的优先级

###24.16_多线程(同步代码块)(掌握)
* 1.什么情况下需要同步
    * 当多线程并发, 有多段代码同时执行时, 我们希望某一段代码执行的过程中CPU不要切换到其他线程工作. 这时就需要同步.
    * 如果两段代码是同步的, 那么同一时间只能执行一段, 在一段代码没执行结束之前, 不会执行另外一段代码.
* 2.同步代码块
    * 使用synchronized关键字加上一个锁对象来定义一段代码, 这就叫同步代码块
    * 多个同步代码块如果使用相同的锁对象, 那么他们就是同步的

class Printer {
                Demo d = new Demo();
                public static void print1() {
                    synchronized(d){                //锁对象可以是任意对象,但是被锁的代码需要保证是同一把锁,不能用匿名对象
                        System.out.print("黑");
                        System.out.print("马");
                        System.out.print("程");
                        System.out.print("序");
                        System.out.print("员");
                        System.out.print("\r\n");
                    }
                }
    
                public static void print2() {    
                    synchronized(d){    
                        System.out.print("传");
                        System.out.print("智");
                        System.out.print("播");
                        System.out.print("客");
                        System.out.print("\r\n");
                    }
                }
            }
###24.17_多线程(同步方法)(掌握)
* 使用synchronized关键字修饰一个方法, 该方法中所有的代码都是同步的

class Printer {
            public static void print1() {
                synchronized(Printer.class){                //锁对象可以是任意对象,但是被锁的代码需要保证是同一把锁,不能用匿名对象
                    System.out.print("黑");
                    System.out.print("马");
                    System.out.print("程");
                    System.out.print("序");
                    System.out.print("员");
                    System.out.print("\r\n");
                }
            }
            /*
             * 非静态同步函数的锁是:this
             * 静态的同步函数的锁是:字节码对象
             */
            public static synchronized void print2() {    
                System.out.print("传");
                System.out.print("智");
                System.out.print("播");
                System.out.print("客");
                System.out.print("\r\n");
            }
        }

###24.18_多线程(线程安全问题)(掌握)
* 多线程并发操作同一数据时, 就有可能出现线程安全问题
* 使用同步技术可以解决这种问题, 把操作数据的代码进行同步, 不要多个线程一起操作
            
            public class Demo2_Synchronized {

/**
                 * @param args
                 * 需求:铁路售票,一共100张,通过四个窗口卖完.
                 */
                public static void main(String[] args) {
                    TicketsSeller t1 = new TicketsSeller();
                    TicketsSeller t2 = new TicketsSeller();
                    TicketsSeller t3 = new TicketsSeller();
                    TicketsSeller t4 = new TicketsSeller();
                    
                    t1.setName("窗口1");
                    t2.setName("窗口2");
                    t3.setName("窗口3");
                    t4.setName("窗口4");
                    t1.start();
                    t2.start();
                    t3.start();
                    t4.start();
                }
            
            }
            
            class TicketsSeller extends Thread {
                private static int tickets = 100;
                static Object obj = new Object();
                public TicketsSeller() {
                    super();
                    
                }
                public TicketsSeller(String name) {
                    super(name);
                }
                public void run() {
                    while(true) {
                        synchronized(obj) {
                            if(tickets <= 0) 
                                break;
                            try {
                                Thread.sleep(10);//线程1睡,线程2睡,线程3睡,线程4睡
                            } catch (InterruptedException e) {
                                
                                e.printStackTrace();
                            }
                            System.out.println(getName() + "...这是第" + tickets-- + "号票");
                        }
                    }
                }
            }

###24.19_多线程(火车站卖票的例子用实现Runnable接口)(掌握)

###24.20_多线程(死锁)(了解)
* 多线程同步的时候, 如果同步代码嵌套, 使用相同锁, 就有可能出现死锁
    * 尽量不要嵌套使用
        
            private static String s1 = "筷子左";
            private static String s2 = "筷子右";
            public static void main(String[] args) {
                new Thread() {
                    public void run() {
                        while(true) {
                            synchronized(s1) {
                                System.out.println(getName() + "...拿到" + s1 + "等待" + s2);
                                synchronized(s2) {
                                    System.out.println(getName() + "...拿到" + s2 + "开吃");
                                }
                            }
                        }
                    }
                }.start();
                
                new Thread() {
                    public void run() {
                        while(true) {
                            synchronized(s2) {
                                System.out.println(getName() + "...拿到" + s2 + "等待" + s1);
                                synchronized(s1) {
                                    System.out.println(getName() + "...拿到" + s1 + "开吃");
                                }
                            }
                        }
                    }
                }.start();
            }

###24.21_多线程(以前的线程安全的类回顾)(掌握)
* A:回顾以前说过的线程安全问题
    * 看源码：Vector,StringBuffer,Hashtable,Collections.synchroinzed(xxx)
    * Vector是线程安全的,ArrayList是线程不安全的
    * StringBuffer是线程安全的,StringBuilder是线程不安全的
    * Hashtable是线程安全的,HashMap是线程不安全的