Java多線程理解

標籤：java   多線程   thread   runnable   

Java實現多線程的兩種方式分別是繼承Thread類和實現Runnable介面。

程式碼範例：

class MyThread extends Thread{ private String name;  public MyThread(String name){  this.name = name; }  @Override   public void run(){  for(int i=0;i<7;i++){   System.out.println("線程開始:"+this.name+"次數"+i);  } }}

class MyRun implements Runnable{ @Override public void run(){  for(int i=0;i<7;i++){   System.out.println("線程開始:"+Thread.currentThread().getName()+"次數"+i);  } }}

public class A{ public static void main(String [] args){   MyThread myThread = new MyThread("myThread");  myThread.start();   MyRun myRun = new MyRun();  Thread run1 = new Thread(myRun, "myRun");  run1.start(); }}

運行結果：

關鍵點1：啟動線程使用start（）方法，使用run（）方法並不會啟動新線程。

public class A{ public static void main(String [] args){   MyThread myThread1 = new MyThread();  myThread1.run();   MyThread myThread2 = new MyThread();  myThread2.run(); }} 

class MyThread extends Thread{ public void run(){  for(int i=0;i<7;i++){   System.out.println("線程:"+Thread.currentThread().getName()+"次數"+i);  } }}

start() : 通過調用start0()啟動一個新線程，新線程會執行相應的run()方法。start()不能被重複調用。

run(): run()和普通的成員方法一樣可重複調用。單獨調用run()會在當前線程中執行run(),並不會啟動新線程！

相關源碼：

public synchronized void start() {        if (threadStatus != 0)  //start()方法不能被重複調用，否則會拋出異常            throw new IllegalThreadStateException();        /* Notify the group that this thread is about to be started         * so that it can be added to the group's list of threads         * and the group's unstarted count can be decremented. */        group.add(this);  //加入到線程組中        boolean started = false;        try {            start0();            started = true;        } finally {            try {                if (!started) {                    group.threadStartFailed(this);                }            } catch (Throwable ignore) {                /* do nothing. If start0 threw a Throwable then                  it will be passed up the call stack */            }        }    }}private native void start0();@Overridepublic void run() {         if (target != null) {   //target是當前的Runnable對象            target.run();        }    }

關鍵點2：Thread實現多線程與Runnable實現多線程的區別.1、由於Java使用單繼承，多介面實現原則，所以通過Runnable介面實現多線程更具靈活性；2、Runnable對象可以被多個線程共用，適合於多個線程處理同一資源的情況。（不準確）

賣票問題：

1：Thread

class MyThread extends Thread{ private int tickets =5;   @Override      public void run() {    while (tickets>0) {              tickets--;            System.out.println(Thread.currentThread().getName()+"賣了一張票，剩餘票數:"+tickets);         }    }}

public static void main(String [] args){   MyThread myThread1 = new MyThread();  myThread1.start();   MyThread myThread2 = new MyThread();  myThread2.start(); }

MyThread myThread1 = new MyThread();MyThread myThread2 = new MyThread();因為每次都建立了新的MyThread對象，所以最後結果是賣了10張票；
2：Runnable

class MyThread extends Thread{ private int tickets =5;   @Override      public void run() {    while (tickets>0) {              tickets--;            System.out.println(Thread.currentThread().getName()+"賣了一張票，剩餘票數:"+tickets);         }    }}

public static void main(String [] args){   MyRunnable r = new MyRunnable();                   Thread t1 = new Thread(r);       Thread t2 = new Thread(r);              t1.start();     t2.start();   }

MyRunnable對象只有一個，多線程共用同一對象，所以賣票結果好像是正確的。

關鍵點3：通過實現Runnable介面共用資源。

class MyRunnable implements Runnable{    private int tickets =10;      @Override      public void run() {       while (tickets>0) {               try {                  Thread.sleep(100);               } catch (InterruptedException e) {                   e.printStackTrace();               }               tickets--;             System.out.println(Thread.currentThread().getName()+"賣了一張票，剩餘票數:"+tickets);          }           }  } 

顯然最後剩餘票數-1和-2是不正確的，所以僅僅實現Runnable介面是無法進行資源共用的，如果需要共用還是需要加入synchronized關鍵字。

class MyRunnable implements Runnable{    private int tickets =10;      @Override      public void run() {       synchronized(this){      while (tickets>0) {                  tickets--;                System.out.println(Thread.currentThread().getName()+"賣了一張票，剩餘票數:"+tickets);             }     }    }}  

Java多線程理解