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[Java教程]synchronized同步引发的思考


  最近公司某同事非常爱学,下班回家后也会抱书学习,看到多线程写例子的时候遇到了非常奇怪的问题,故而将例子发给我看让给解答,下面给出例子。

  1.第一例及运行结果

    下面是示例代码

 1 package com.coderweb.sys.util; 2  3 public class TxtThread implements Runnable { 4  5   Integer num = 10000; 6   String str = new String(); 7  8   @Override 9   public void run() {10     synchronized (num) {11       while (true) {12         if (num > 0) {13           try {14             // Thread.sleep(10);15           } catch (Exception e) {16             e.getMessage();17           }18           System.out.println(Thread.currentThread().getName()19               + " this is " + num--);20 21           // str+="1";22         } else {23           break;24         }25 26       }27     }28   }29 30   public static void main(String[] args) {31     TxtThread tt = new TxtThread();32     new Thread(tt).start();33     new Thread(tt).start();34     new Thread(tt).start();35     new Thread(tt).start();36   }37 }

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     下面是运行截图的一部分

Thread-0 this is 10000
Thread-2 this is 9999
Thread-0 this is 9998
Thread-2 this is 9997
Thread-3 this is 9995
Thread-0 this is 9996
Thread-3 this is 9993
Thread-3 this is 9991
Thread-3 this is 9990
Thread-3 this is 9989
Thread-3 this is 9988
Thread-3 this is 9987
Thread-3 this is 9986
Thread-3 this is 9985

   问题一:Integer不是对象吗?对象引用不变的呀?修改值后应该不变的呀?为啥起了四个线程后,四个线程都对一个对象里的值进行修改了呢?为啥synchronized语句块没有起到任何作用呢?

   带着问题一,修改例子

   2.第二例及运行结果(唯一不同的地方是,synchronized(中的内容))

 1 package com.coderweb.sys.util; 2  3 public class TxtThread implements Runnable { 4  5   Integer num = 10000; 6   String str = new String(); 7  8   @Override 9   public void run() {10     synchronized (str) {11       while (true) {12         if (num > 0) {13           try {14             // Thread.sleep(10);15           } catch (Exception e) {16             e.getMessage();17           }18           System.out.println(Thread.currentThread().getName()19               + " this is " + num--);20 21           // str+="1";22         } else {23           break;24         }25 26       }27     }28   }29 30   public static void main(String[] args) {31     TxtThread tt = new TxtThread();32     new Thread(tt).start();33     new Thread(tt).start();34     new Thread(tt).start();35     new Thread(tt).start();36   }37 }

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    运行结果部分

Thread-0 this is 10000
Thread-0 this is 9999
Thread-0 this is 9998
Thread-0 this is 9997
Thread-0 this is 9996
Thread-0 this is 9995

.........

Thread-0 this is 5
Thread-0 this is 4
Thread-0 this is 3
Thread-0 this is 2
Thread-0 this is 1

通过问题2,可以得出总结,对于String字符串在初始化后,其引用地址没有发生变化,后面也没有进行修改,因此多个线程同时访问的时候起到了互斥的作用,当四个线程启动后,哪个线程先进入代码块进行了加锁,谁将一直持有该锁直到该线程结束,其余线程发现有线程持有该string的锁,将处于等待状态,因此结论便是,这四个线程,哪个线程先进入同步快,将一直打印该线程的数据。

           由问题1跟2的不同运行结果发现,区别之处在于第一例子中的synchronized是num,并在后面进行了减法操作,而第二个例子中的synchronized是str,并且该str没有发生变化,难道是因为num改变之后引用地址发生变化了?下面给出思考问题的验证例子3跟4

3.第三例及运行结果

package com.coderweb.sys.util;public class TxtThread implements Runnable {  Integer num = 10000;  String str = new String();  Integer testI = 0;  @Override  public void run() {    synchronized (testI) {      while (true) {        if (num > 0) {          try {            // Thread.sleep(10);          } catch (Exception e) {            e.getMessage();          }          System.out.println(Thread.currentThread().getName()              + " this is " + num--);          // str+="1";        } else {          break;        }      }    }  }  public static void main(String[] args) {    TxtThread tt = new TxtThread();    new Thread(tt).start();    new Thread(tt).start();    new Thread(tt).start();    new Thread(tt).start();  }}

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    运行结果

Thread-0 this is 10000
Thread-0 this is 9999
Thread-0 this is 9998
Thread-0 this is 9997
Thread-0 this is 9996
Thread-0 this is 9995
Thread-0 this is 9994
Thread-0 this is 9993

。。。。。。

Thread-0 this is 7
Thread-0 this is 6
Thread-0 this is 5
Thread-0 this is 4
Thread-0 this is 3
Thread-0 this is 2
Thread-0 this is 1

  该例子的不同之处在于,新加了一个成员变量testI,并且没有对该值进行操作,发现结果居然成功,只有一个线程持有锁,这也就验证了Integer类型的确是引用类型,在

创建完成后的引用地址没有发生变化。那么猜想string如果内容变了会怎样呢?例子4进行验证

  4.第四例及运行结果

 1 package com.coderweb.sys.util; 2  3 public class TxtThread implements Runnable { 4  5   Integer num = 10000; 6   String str = new String(); 7  8   @Override 9   public void run() {10     synchronized (str) {11       while (true) {12         if (num > 0) {13           try {14             // Thread.sleep(10);15           } catch (Exception e) {16             e.getMessage();17           }18           System.out.println(Thread.currentThread().getName()19               + " this is " + num--);20 21           str+="1";22         } else {23           break;24         }25 26       }27     }28   }29 30   public static void main(String[] args) {31     TxtThread tt = new TxtThread();32     new Thread(tt).start();33     new Thread(tt).start();34     new Thread(tt).start();35     new Thread(tt).start();36   }37 }

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  运行部分结果

     .............................

Thread-3 this is 9774
Thread-2 this is 9779
Thread-3 this is 9773
Thread-0 this is 9777
Thread-3 this is 9771
Thread-3 this is 9769
Thread-3 this is 9768
Thread-2 this is 9772
Thread-3 this is 9767
Thread-0 this is 9770
Thread-3 this is 9765
Thread-2 this is 9766
Thread-3 this is 9763
Thread-0 this is 9764
Thread-3 this is 9761

..............................

该例子的不同之处在于,在循环最后不停的对str进行修改,所以导致了多个线程同时访问,并没有起到加锁的作用。

 

但是我们的都知道,string类型变量是不可变的,也就是所说的immutable,就是说在对象创建之后,该string的引用类型变量是不变的,如果对该变量进行修改操作之后,会重新建立对象,并将新对象的地址赋给该引用,也就是说例子中的不停的修改str对象就相当于不停的创建新对象并赋给该引用。这个例子还好理解,毕竟我们对string还稍微有点了解,但是为什么Integer也会有这样的效果呢,难道我们对Integer进行了修改之后起引用地址也发生了变化?下面查看了jdk关于Integer封装类的源码

 

5.JDK中关于Integer的部分源码

public final class Integer extends Number implements Comparable<Integer> {     /**   * The value of the <code>Integer</code>.   *   * @serial   */  private final int value;/**   * Compares this object to the specified object. The result is   * <code>true</code> if and only if the argument is not   * <code>null</code> and is an <code>Integer</code> object that   * contains the same <code>int</code> value as this object.   *   * @param  obj  the object to compare with.   * @return <code>true</code> if the objects are the same;   *     <code>false</code> otherwise.   */  public boolean equals(Object obj) {  if (obj instanceof Integer) {    return value == ((Integer)obj).intValue();  }  return false;  } /**   * Returns a hash code for this <code>Integer</code>.   *   * @return a hash code value for this object, equal to the   *     primitive <code>int</code> value represented by this   *     <code>Integer</code> object.   */  public int hashCode() {  return value;  }

  观察上面的源码我们便能明白道理了,在Integer封装类中,利用了一个final的int类型,也就是说一旦对象创建,该值便不能改变了,但是为啥我们还能对其进行修改呢,所以必定是我们修改了之后,会创建新的地址,并赋给新的引用,我们先通过下面例子验证一把是否引用地址发生了变化

  

 1 public static void main(String[] args) { 2 //    TxtThread tt = new TxtThread(); 3 //    new Thread(tt).start(); 4 //    new Thread(tt).start(); 5 //    new Thread(tt).start(); 6 //    new Thread(tt).start(); 7      8     Integer number = 5; 9     Integer number2 = number;10     number2--;11     System.out.println("number---"+number);12     System.out.println("number2---"+number2);13     System.out.println("number ==number2? "+(number==number2));14   }

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  这个例子中,我们定义了第一个对象,这个时候第一个对象地址没有发生变化,这时我们创建了新对象,并指向第一个对象,这时候两个对象的引用地址是一样的,紧接着我们对第二个对象进行了修改,当然其值是发生了变化,其实我们可以想一下,如果地址没有发生变化的话,5是怎么等于4的呢?所以地址必然不一样,最后的false也就验证了这一点。当然咱通过Integer的源代码发现,其equals方法也是通过判断其中的值类判断两个Integer是否相等的。

    综上所有事例得出结论:Integer这类对于基本数据类型的封装类,当其值发生改变时,其引用地址也发生了变化。