線程八大核心【一】

1.建立線程的方式

前面我看過不少的博客，部分有說建立線程有多種方式，但JDK1.8源碼Thread中有這麼一段話.

There are two ways to create a new thread of execution.One is to declare a class to be a subclass of Thread,The other way to create a thread is to declare a class that implements the Runnable interface.

官方給出的說明是本質上只有兩種方式：1.繼承Thread類，2.實現Runnable接口。但還有不少外在的表現形式，例如lambda表達式，線程池等。

public class NewThread {
    /*擴展自Thread類*/
    private static class UseThread extends Thread{
        @Override
        public void run() {
            super.run();
            // do my work;
            System.out.println("I am extendec Thread");
        }
    }
    /*實現Runnable接口*/
    private static class UseRunnable implements Runnable{

        @Override
        public void run() {
            // do my work;
            System.out.println("I am implements Runnable");
        }
    }
    
    public static void main(String[] args) 
            throws InterruptedException, ExecutionException {
        UseThread useThread = new UseThread();
        useThread.start();
        UseRunnable useRunnable = new UseRunnable();
        new Thread(useRunnable).start();
    }

2.中止線程的正確姿式

2.1 繼承Thread，執行方法體中沒有InterruptException

直接上代碼：

public class EndThread {
    private static class UseThread extends Thread{
        public UseThread(String name) {
            super(name);
        }
        @Override
        public void run() {
            String threadName = Thread.currentThread().getName();
            System.out.println(threadName+" interrrupt flag ="+isInterrupted());
            while(!isInterrupted()){
                //while(!Thread.interrupted()){
                System.out.println(threadName+" is running");
                System.out.println(threadName+"inner interrrupt flag ="
                        +isInterrupted());
            }
            System.out.println(threadName+" interrrupt flag ="+isInterrupted());
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread endThread = new UseThread("endThread");
        endThread.start();
        Thread.sleep(20);
        endThread.interrupt();//中斷線程，其實設置線程的標識位true
    }
}

2.1.1 說明：

stop()、suspend()也能中止線程，那麼爲何要將其標註爲Deprecated(過期)的方法呢？緣由就是這種方式是搶佔式的，好比：有一個任務在寫文件，文件的總大小爲10M，如今寫了5M，忽然收到stop()的中斷，致使線程忽然中止，那麼這樣就破壞了文件的完整性。使用interrupt()方法的好處就是，調用該方法時會給線程的中斷標誌位設置爲true，並把中斷線程的決定權交給線程本身，這種方式就是協做式，等任務處理完再結束。

2.1.2 isInterrupt()與Thread.interrupt()靜態方法的區別

相同點：調用interrupt()方法時，二者都能獲取線程的中斷標誌位狀態爲true
不一樣點：Thread.interrupt()在判斷完以後會將中斷標誌位重置爲false

2.2 實現Runnable，執行方法體中沒有InterruptException

public class EndRunnable {
    private static class UseRunnable implements Runnable{
        @Override
        public void run() {
            while(!Thread.currentThread().isInterrupted()) {
                System.out.println(Thread.currentThread().getName()
                        + " I am implements Runnable.");
            }
            System.out.println(Thread.currentThread().getName()
                    +" interrupt flag is "+Thread.currentThread().isInterrupted());
        }
    }
    public static void main(String[] args) throws InterruptedException {
        UseRunnable useRunnable = new UseRunnable();
        Thread endThread = new Thread(useRunnable,"endThread");
        endThread.start();
        Thread.sleep(20);
        endThread.interrupt();
    }
}

2.3 執行方法體中有InterruptException

當線程的執行體中出現了sleep、await、join等會拋出interruptException異常的方法時，須要特別注意，就例以下面的程序。線程調用interrupt方法時，線程的執行體會捕獲到interruptException異常，進入到catch中，這時會將線程的中斷標誌位重置爲false。這樣作的意義在於：若是不重置爲false的話，線程收到中斷請求立馬就結束了那麼與stop方法沒有區別了，若是重置爲false，程序能夠在catch中作未完成的工做，如釋放資源等，作完這些收尾的工做以後再手動的調用一次interrupt()方法，結束線程。

/**
 *類說明：阻塞方法中拋出InterruptedException異常後，若是須要繼續中斷，須要手動再中斷一次
 */
public class HasInterrputException {
    private static class UseThread extends Thread{
        public UseThread(String name) {
            super(name);
        }
        @Override
        public void run() {
            while(!isInterrupted()) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    System.out.println(Thread.currentThread().getName()
                            +" in InterruptedException interrupt flag is "
                            +isInterrupted());
                    //資源釋放
                    //interrupt();
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()
                        + " I am extends Thread.");
            }
            System.out.println(Thread.currentThread().getName()
                    +" interrupt flag is "+isInterrupted());
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread endThread = new UseThread("HasInterrputEx");
        endThread.start();
        Thread.sleep(500);
        endThread.interrupt();    
    }
}

2.4 錯誤的結束線程方式

設置標誌標誌位，經過判斷標誌位來中斷線程，這種方式的不合理因素在於，若是線程執行體中的方法長時間被阻塞，致使循環阻塞，中斷標誌位失效。

/**
 *類說明：設置標誌位中斷線程
 */
public class HasInterrputException {
    
    private static class UseThread extends Thread{
        private volatile boolean flag = false;

        public UseThread(String name) {
            super(name);
        }
        public void setFlag(boolean flag) {
            this.flag = flag;
        }
        @Override
        public void run() {
            while(!flag) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    System.out.println(Thread.currentThread().getName()
                            +" in InterruptedException interrupt flag is "
                            +isInterrupted());
                    //資源釋放
                    //interrupt();
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()
                        + " I am extends Thread.");
            }
            System.out.println(Thread.currentThread().getName()
                    +" interrupt flag is "+isInterrupted());
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread endThread = new UseThread("HasInterrputEx");
        endThread.start();
        Thread.sleep(500);
        ((UseThread) endThread).setFlag(true);
    }
}

3.線程的狀態轉化

jdk源碼對Thread類定義了6個狀態，分別爲:

public enum State {
        
        NEW,

        RUNNABLE,
        
        BLOCKED,

        WAITING,

        TIMED_WAITING,

        TERMINATED;
    }

4.Thread類的主要方法