java中多线程

介绍一下线程中基本的方法使用

线程睡眠sleep()

Thread.sleep(毫秒);我们可以通过sleep方法设置让线程睡眠。可以看到sleep是个静态方法

public static native void sleep(long var0) throws InterruptedException;

    try {
        System.out.println(new Date().getSeconds());
        Thread.sleep(5000);
        System.out.println(new Date().getSeconds());
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

setDaemon守护线程

非守护线程停止，那么守护线程自动退出

    public static void main(String[] args) {
        Thread thread1 = new Thread() {
            @Override
            public void run() {
                super.run();
                for(int i = 0; i < 5; i ++) {
                    System.out.println("非守护线程");
                }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for(int i = 0; i < 200; i ++) {
                    System.out.println("守护线程");
                }
            }
        };

        thread2.setDaemon(true);
        thread1.start();
        thread2.start();
    }

可以很明显的看到thread2本应该执行200次输出，但是这里只输出了几行。因为当thread1执行完毕后，thread2作为守护线程就自动停止了。

多线程join

如果执行了join方法，那么停止当前线程，先跑执行了join()的线程。相当于插队执行。如下，在执行thread2线程的时候，如果i==20的时候，则让thread1插队先执行

    public static void main(String[] args) {
        final Thread thread1 = new Thread() {
            @Override
            public void run() {
            super.run();
            for(int i = 0; i < 500; i ++) {
                System.out.println("thread1---" + i);
            }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for(int i = 0; i < 200; i ++) {
                    if (i == 20) {
                        try {
                            //插队执行
                            thread1.join();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(i);
                }
            }
        };
        thread1.start();
        thread2.start();
    }

join()方法也可以传参数long 毫秒 join(毫秒)
表示让执行join的线程，插队执行XXX毫秒，过了时间后，两个线程交替执行

   public static void main(String[] args) {
        final Thread thread1 = new Thread() {
            @Override
            public void run() {
            super.run();
            for(int i = 0; i < 500; i ++) {
                System.out.println("thread1---" + i);
            }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for(int i = 0; i < 200; i ++) {
                    if (i == 20) {
                        try {
                            //插队执行１毫秒
                            thread1.join(1);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(i);
                }
            }
        };
        thread1.start();
        thread2.start();
    }

yeild 礼让线程

yeild会让出cpu,让其他线程执行

    public static void main(String[] args) {
        final Thread thread1 = new Thread() {
            @Override
            public void run() {
            super.run();
            for(int i = 0; i < 500; i ++) {
                System.out.println( getName() + "---" + i);
            }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for(int i = 0; i < 200; i ++) {
                    if (i % 5 == 0) {
                        Thread.yield();
                    }
                    System.out.println(getName() + "---" + i);
                }
            }
        };
        thread1.start();
        thread2.start();
    }

setPriority给线程设置优先级

默认优先级是5 最小１，最大10

越大优先级越高

    public static void main(String[] args) {
        final Thread thread1 = new Thread() {
            @Override
            public void run() {
            super.run();
            for(int i = 0; i < 500; i ++) {
                System.out.println( getName() + "---" + i);
            }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for(int i = 0; i < 500; i ++) {

                    System.out.println(getName() + "---" + i);
                }
            }
        };
        //设置最大的线程优先级最大为10
        thread1.setPriority(Thread.MIN_PRIORITY);
        //设置最小的线程优先级，最小为１
        thread2.setPriority(Thread.MAX_PRIORITY);
        thread1.start();
        thread2.start();
    }

synchronized

同步代码块

当多线程并发，多段代码同时执行的时候。希望在执行其中代码的时候，cpu不切换线程

不用synchronized的情况

我们来看一下不用synchronized的情况会发生什么

public class ThreadSynchronied {

    public static void main(String[] args) {
        final Say say = new Say();

         Thread thread1 = new Thread() {
            @Override
            public void run() {
                for (int i = 0 ; i < 10000 ; i ++) {
                    say.say();
                }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for (int i = 0 ; i < 10000 ; i ++) {
                    say.say1();
                }
            }
        };
        //设置最大的线程优先级最大为10
        thread1.setPriority(Thread.MIN_PRIORITY);
        //设置最小的线程优先级，最小为１
        thread2.setPriority(Thread.MAX_PRIORITY);
        thread1.start();
        thread2.start();
    }
}

class Say {
    void say() {
        System.out.print("s ");
        System.out.print("a ");
        System.out.print("y ");
        System.out.print("h ");
        System.out.print("e ");
        System.out.print("l ");
        System.out.print("l ");
        System.out.println("o");
    }

    void say1() {
        System.out.print("1 ");
        System.out.print("2 ");
        System.out.print("3 ");
        System.out.print("4 ");
        System.out.print("5 ");
        System.out.print("6 ");
        System.out.print("7 ");
        System.out.println("8");
    }
}

我们发现有些输出并没有打印全，在执行线程thread1的过程中，cpu被thread2抢占。这种情况下，肯定是不符合我们的业务逻辑的。所以我们要保证线程执行了一个完整的方法后，cpu才会被其他线程抢占

使用synchronized

public class ThreadSynchronied {

    public static void main(String[] args) {
        final Say say = new Say();

         Thread thread1 = new Thread() {
            @Override
            public void run() {
                for (int i = 0 ; i < 10000 ; i ++) {
                    say.say();
                }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for (int i = 0 ; i < 10000 ; i ++) {
                    say.say1();
                }
            }
        };
        //设置最大的线程优先级最大为10
        thread1.setPriority(Thread.MIN_PRIORITY);
        //设置最小的线程优先级，最小为１
        thread2.setPriority(Thread.MAX_PRIORITY);
        thread1.start();
        thread2.start();
    }
}

class Say {
    String s = "hahaah";

    void say() {
        synchronized (s) {
            System.out.print("s ");
            System.out.print("a ");
            System.out.print("y ");
            System.out.print("h ");
            System.out.print("e ");
            System.out.print("l ");
            System.out.print("l ");
            System.out.println("o");
        }
    }

    void say1() {
        synchronized (s) {
            System.out.print("1 ");
            System.out.print("2 ");
            System.out.print("3 ");
            System.out.print("4 ");
            System.out.print("5 ");
            System.out.print("6 ");
            System.out.print("7 ");
            System.out.println("8");
        }
    }
}

使用synchronized同步代码块后，就发现不会出现上述情况了

同步方法

public class ThreadSynchroniedMethod {

    public static void main(String[] args) {
        final Say say = new Say();

         Thread thread1 = new Thread() {
            @Override
            public void run() {
                for (int i = 0 ; i < 10000 ; i ++) {
                    say.say();
                }
            }
        };

        Thread thread2 = new Thread() {
            @Override
            public void run() {
                for (int i = 0 ; i < 10000 ; i ++) {
                    say.say1();
                }
            }
        };
        //设置最大的线程优先级最大为10
        thread1.setPriority(Thread.MIN_PRIORITY);
        //设置最小的线程优先级，最小为１
        thread2.setPriority(Thread.MAX_PRIORITY);
        thread1.start();
        thread2.start();
    }
}

class Say {
    //在方法上加锁
    static synchronized void say() {
            System.out.print("s ");
            System.out.print("a ");
            System.out.print("y ");
            System.out.print("h ");
            System.out.print("e ");
            System.out.print("l ");
            System.out.print("l ");
            System.out.println("o");

    }

     static void say1() {
        synchronized (Say.class) {
            System.out.print("1 ");
            System.out.print("2 ");
            System.out.print("3 ");
            System.out.print("4 ");
            System.out.print("5 ");
            System.out.print("6 ");
            System.out.print("7 ");
            System.out.println("8");
        }
    }
}

同步方法指的就是在方法上加锁

静态同步方法的所对象是该类的字节码对象
非静态的同步方法锁对象是this

多个线程使用同一资源锁，容易造成死锁

什么是死锁?

死锁是指两个或两个以上的进程在执行过程中，由于竞争资源或者由于彼此通信而造成的一种阻塞的现象，若无外力作用，它们都将无法推进下去。此时称系统处于死锁状态或系统产生了死锁，这些永远在互相等待的进程称为死锁进程。

线程安全类

Vector
StringBuffer
HashTable

线程不安全

ArrayList
StringBuilder
HashSet

java.util.Collections中有synchronizedList等方法，支持我们把线程不安全的集合转成线程安全的

学习笔记

多次启动一个线程是非法的