Java 同步 : Synchronization method and synchronization block

Question

我有一个多线程 Java 应用程序。在一种方法中，需要同步一个 ArrayList。由于 arrayList 不是线程安全的，所以我必须使用同步。问题是 ArrayList 类型的对象不是对象的成员变量。方法原型(prototype)如下:

public void simultaneousAccess(ArrayListWrapper aListWrapper){
  ArrayList list = aListWrapper.getList();
  //...Codes manipulate the list            
}

由于多线程，我应该使用

一个)

 public void synchronized simultaneousAccess(ArrayListWrapper aListWrapper){
         ArrayList list = aListWrapper.getList();
         //...Codes manipulate the list            
    }

或者

B)

public void simultaneousAccess(ArrayListWrapper aListWrapper){
     ArrayList list = aListWrapper.getList();
     Synchronized(list){
         //...Codes manipulate the list 
     }           
 }

从性能测试来看，两者都不起作用。但我不知道为什么？

这里是完整的源代码:

package com.juhani.prototype.sync;

import java.util.ArrayList;

public class ArrayListWrapper {
    public ArrayList<Integer> aList = new ArrayList<Integer>();

    public ArrayListWrapper(){
        Integer one = new Integer(1);
        Integer two = new Integer(2);
        Integer three = new Integer(3);

        aList.add(one);
        aList.add(two);
        aList.add(three);
    }
}

package com.juhani.prototype.sync;

import java.util.ArrayList;

public class TestClass {

    public int count_test=0;

    public synchronized void test(ArrayListWrapper listWrapper){        
        ArrayList<Integer> list = listWrapper.aList;  
        int temp = list.get(1)+1;
        list.set(1,temp);       
    }

    public void testBlock(ArrayListWrapper listWrapper){
        ArrayList<Integer> list = listWrapper.aList;  
        synchronized(list){
            int temp = list.get(1)+1;
            list.set(1,temp);     
        }
    }

}


package com.juhani.prototype.sync;

public class WorkerSyncObj extends Thread {

    ArrayListWrapper listWrapper = null;
    TestClass tc = null;
    int number;

    public WorkerSyncObj(int aNumber){
       number = aNumber;    
    }

    public void setListWrapper(ArrayListWrapper aListWrapper){
        listWrapper = aListWrapper;
    }

    public void setTestClass(TestClass aTc){
        tc = aTc;
    }

    public void run(){
        int i = 1000;
        for(int j=0;j<i;j++){
            tc.testBlock(listWrapper);
            System.out.println("Thread "+number+" is runing at loop "+j+" . index 1 value is:"+listWrapper.aList.get(1)); 
        }       
    }   
}

package com.juhani.prototype.sync.main;

import com.juhani.prototype.sync.ArrayListWrapper;
import com.juhani.prototype.sync.TestClass;
import com.juhani.prototype.sync.WorkerSyncObj;

public class TestMain {

    public static void main(String[] args){

        ArrayListWrapper list = new ArrayListWrapper();
        TestClass tc = new TestClass();

        WorkerSyncObj work1 = new WorkerSyncObj(1);
        work1.setListWrapper(list);
        work1.setTestClass(tc);
        WorkerSyncObj work2 = new WorkerSyncObj(2);
        work2.setListWrapper(list);
        work2.setTestClass(tc);
        WorkerSyncObj work3 = new WorkerSyncObj(3);
        work3.setListWrapper(list);
        work3.setTestClass(tc);

        work1.start();
        work2.start();
        work3.start();



    }

}

Answer 1

在第一种情况下，您锁定了 this 对象，而在第二种情况下，您锁定了 list 对象。如果您从不同的对象调用该方法但列表相同，则这可能是一个问题。这可能是第一种情况下出现异常的原因。
或者，您可以尝试一些内置的并发类型，例如 Collections.synchronizedList 或 CopyOnWriteArrayList。