Java多線程理解

2019-11-14 15:37:01

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一、理論概念

進程與線程

進程（PRocess）是計算機中的程序關于某數據集合上的一次運行活動，是系統進行資源分配和調度的基本單位。

線程是進程的一個實體,是CPU調度和分派的基本單位,它是比進程更小的能獨立運行的基本單位.線程自己基本上不擁有系統資源,只擁有一點在運行中必不可少的資源(如程序計數器,一組寄存器和棧),但是它可與同屬一個進程的其他的線程共享進程所擁有的全部資源.

進程有獨立的地址空間，一個進程崩潰后，在保護模式下不會對其它進程產生影響，而線程只是一個進程中的不同執行路徑。線程有自己的堆棧和局部變量，但線程之間沒有單獨的地址空間，一個線程死掉就等于整個進程死掉，所以多進程的程序要比多線程的程序健壯，但在進程切換時，耗費資源較大，效率要差一些。但對于一些要求同時進行并且又要共享某些變量的并發操作，只能用線程，不能用進程。

二、線程的java實現

目前實現java線程有2種方式，繼承Thread類和實現Runnable接口，重寫run方法。

1、實現Runnable接口

實現 Runnable的接口的好處是實現變量的共享，多個線程使用同一個Runable對象，如上代碼塊中的成員變量j：

package src;/** *  * @author Shawn Wang *  */public class MyThread implements Runnable {    private int j = 100;    @Override    public void run() {        while (true) {            if (j > 0) {                System.out.println(Thread.currentThread().getName() + "-- num:"                        + j--);            } else {                break;            }        }    }    public static void main(String[] args) {        MyThread mt = new MyThread();        Thread thread1 = new Thread(mt);        Thread thread2 = new Thread(mt);        Thread thread3 = new Thread(mt);        thread1.start();        thread2.start();        thread3.start();    }}

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輸出結果：

Thread-1-- num:100Thread-3-- num:98Thread-2-- num:99Thread-2-- num:95Thread-3-- num:96Thread-1-- num:97Thread-1-- num:92Thread-1-- num:91Thread-3-- num:93Thread-3-- num:89Thread-3-- num:88Thread-2-- num:94Thread-2-- num:86Thread-2-- num:85Thread-2-- num:84Thread-3-- num:87Thread-3-- num:82Thread-1-- num:90Thread-1-- num:80Thread-3-- num:81Thread-3-- num:78Thread-3-- num:77Thread-3-- num:76Thread-3-- num:75Thread-3-- num:74Thread-2-- num:83Thread-2-- num:72Thread-2-- num:71Thread-2-- num:70Thread-3-- num:73Thread-1-- num:79Thread-3-- num:68Thread-3-- num:66Thread-3-- num:65Thread-2-- num:69Thread-2-- num:63Thread-2-- num:62Thread-2-- num:61Thread-2-- num:60Thread-2-- num:59Thread-2-- num:58Thread-2-- num:57Thread-2-- num:56Thread-2-- num:55Thread-2-- num:54Thread-2-- num:53Thread-2-- num:52Thread-3-- num:64Thread-1-- num:67Thread-3-- num:50Thread-2-- num:51Thread-3-- num:48Thread-1-- num:49Thread-3-- num:46Thread-2-- num:47Thread-3-- num:44Thread-1-- num:45Thread-3-- num:42Thread-2-- num:43Thread-3-- num:40Thread-1-- num:41Thread-3-- num:38Thread-2-- num:39Thread-3-- num:36Thread-1-- num:37Thread-3-- num:34Thread-2-- num:35Thread-3-- num:32Thread-1-- num:33Thread-3-- num:30Thread-2-- num:31Thread-3-- num:28Thread-1-- num:29Thread-3-- num:26Thread-2-- num:27Thread-3-- num:24Thread-1-- num:25Thread-3-- num:22Thread-2-- num:23Thread-3-- num:20Thread-1-- num:21Thread-3-- num:18Thread-2-- num:19Thread-3-- num:16Thread-1-- num:17Thread-3-- num:14Thread-2-- num:15Thread-3-- num:12Thread-1-- num:13Thread-3-- num:10Thread-2-- num:11Thread-3-- num:8Thread-1-- num:9Thread-3-- num:6Thread-2-- num:7Thread-3-- num:4Thread-1-- num:5Thread-3-- num:2Thread-2-- num:3Thread-1-- num:1

result

2、繼承Thread類

package src;public class ThreadClient extends Thread {    public ThreadClient(String name) {        super(name);    }    @Override    public void run() {        // TODO Auto-generated method stub        for (int i = 0; i <= 10; i++) {            System.out.println(currentThread().getName() + ":" + i);        }    }       public static void main(String[] args) {        ThreadClient thread1 = new ThreadClient("thread1");        ThreadClient thread2 = new ThreadClient("thread1");        ThreadClient thread3 = new ThreadClient("thread1");        thread1.start();        thread2.start();        thread3.start();    }}

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輸出結果

thread1:0thread1:1thread1:0thread1:1thread1:0thread1:1thread1:2thread1:3thread1:4thread1:5thread1:6thread1:2thread1:2thread1:3thread1:4thread1:3thread1:4thread1:5thread1:6thread1:7thread1:7thread1:8thread1:9thread1:10thread1:5thread1:6thread1:7thread1:8thread1:9thread1:8thread1:9thread1:10thread1:10

result

疑問：

為什么要重寫 run方法？

run方法時存儲線程所要運行的代碼，不能在直接調用run方法，需要通過start方法來調用。

三、線程同步

場景：如果線程進行了sleep 操作時，其他線程對共享變量做了改變，會造成數據不同步問題。

package src;/** *  * @author Shawn Wang *  */public class MyThread implements Runnable {    private int j = 100;    @Override    public void run() {        while (true) {            if (j > 0) {                try {                    Thread.sleep(1000);// 當前線程休眠1秒                } catch (Exception e) {                    e.printStackTrace();                }                System.out.println(Thread.currentThread().getName() + "-- num:"                        + j--);            } else {                break;            }        }    }    public static void main(String[] args) {        MyThread mt = new MyThread();        Thread thread1 = new Thread(mt);        Thread thread2 = new Thread(mt);        Thread thread3 = new Thread(mt);        thread1.start();        thread2.start();        thread3.start();    }}

View Code

輸出結果：

Thread-1-- num:100Thread-2-- num:99Thread-3-- num:98Thread-1-- num:97Thread-3-- num:95Thread-2-- num:96Thread-1-- num:94Thread-2-- num:92Thread-3-- num:93Thread-3-- num:91Thread-2-- num:90Thread-1-- num:90Thread-3-- num:89Thread-2-- num:88Thread-1-- num:87Thread-3-- num:86Thread-2-- num:85Thread-1-- num:85Thread-3-- num:84Thread-2-- num:83Thread-1-- num:82Thread-3-- num:81Thread-1-- num:80Thread-2-- num:80Thread-3-- num:79Thread-2-- num:77Thread-1-- num:78Thread-3-- num:76Thread-2-- num:75Thread-1-- num:74Thread-2-- num:73Thread-3-- num:71Thread-1-- num:72Thread-2-- num:70Thread-1-- num:69Thread-3-- num:69Thread-2-- num:68Thread-3-- num:66Thread-1-- num:67Thread-2-- num:65Thread-1-- num:64Thread-3-- num:64Thread-2-- num:63Thread-1-- num:62Thread-3-- num:61Thread-2-- num:60Thread-3-- num:58Thread-1-- num:59Thread-2-- num:57Thread-1-- num:56Thread-3-- num:56Thread-2-- num:55Thread-1-- num:54Thread-3-- num:54Thread-2-- num:53Thread-3-- num:52Thread-1-- num:52Thread-3-- num:51Thread-2-- num:50Thread-1-- num:51Thread-3-- num:49Thread-1-- num:48Thread-2-- num:48Thread-3-- num:47Thread-1-- num:46Thread-2-- num:45Thread-1-- num:44Thread-2-- num:42Thread-3-- num:43Thread-3-- num:40Thread-1-- num:39Thread-2-- num:41Thread-2-- num:38Thread-3-- num:37Thread-1-- num:38Thread-1-- num:36Thread-2-- num:35Thread-3-- num:36Thread-1-- num:34Thread-2-- num:33Thread-3-- num:33Thread-1-- num:32Thread-3-- num:30Thread-2-- num:31Thread-1-- num:29Thread-3-- num:29Thread-2-- num:29Thread-3-- num:28Thread-1-- num:28Thread-2-- num:28Thread-1-- num:27Thread-2-- num:26Thread-3-- num:27Thread-1-- num:25Thread-2-- num:24Thread-3-- num:24Thread-1-- num:23Thread-3-- num:22Thread-2-- num:22Thread-1-- num:21Thread-3-- num:20Thread-2-- num:20Thread-1-- num:19Thread-3-- num:18Thread-2-- num:17Thread-3-- num:16Thread-1-- num:15Thread-2-- num:14Thread-2-- num:13Thread-3-- num:12Thread-1-- num:13Thread-2-- num:11Thread-1-- num:10Thread-3-- num:10Thread-2-- num:9Thread-1-- num:7Thread-3-- num:8Thread-2-- num:6Thread-3-- num:5Thread-1-- num:5Thread-2-- num:4Thread-3-- num:3Thread-1-- num:2Thread-2-- num:1Thread-1-- num:0Thread-3-- num:-1

result

出現了負數。

解決加上同步鎖synchronized

同步代碼塊的格式：
synchronized(對象){
需要被同步的代碼;
}

package src;/** *  * @author Shawn Wang *  */public class MyThread implements Runnable {    private int j = 100;    @Override    public void run() {        while (true) {            synchronized (this) {                if (j > 0) {                    try {                        Thread.sleep(1000);// 當前線程休眠1秒                    } catch (Exception e) {                        e.printStackTrace();                    }                    System.out.println(Thread.currentThread().getName()                            + "-- num:" + j--);                } else {                    break;                }            }        }    }    public static void main(String[] args) {        MyThread mt = new MyThread();        Thread thread1 = new Thread(mt);        Thread thread2 = new Thread(mt);        Thread thread3 = new Thread(mt);        thread1.start();        thread2.start();        thread3.start();    }}