Synchronizing with threads. 1) NEVER use synchronized(this) in a thread it doesn't work. Synchronizing with (this) uses the current thread as the locking thread object. Since each thread is independent of other threads, there is NO coordination of synchronization. 2) Tests of code show that in Java 1.6 on a Mac the method synchronization does not work. 3) synchronized(lockObj) where lockObj is a common shared object of all threads synchronizing on it will work. 4) ReenterantLock.lock() and .unlock() work. See Java tutorials for this.

The following code shows these points. It also contains the thread-safe Vector which would be substituted for the ArrayList, to show that many threads adding to a Vector do not lose any information, while the same with an ArrayList can lose information. 0) Current code shows loss of information due to race conditions A) Comment the current labeled A line, and uncomment the A line above it, then run, method loses data but it shouldn't. B) Reverse step A, uncomment B and // end block }. Then run to see results no loss of data C) Comment out B, uncomment C. Run, see synchronizing on (this) loses data, as expected. Don't have time to complete all the variations, hope this helps. If synchronizing on (this), or the method synchronization works, please state what version of Java and OS you tested. Thank you.

import java.util.*;

/** RaceCondition - Shows that when multiple threads compete for resources 
     thread one may grab the resource expecting to update a particular 
     area but is removed from the CPU before finishing.  Thread one still 
     points to that resource.  Then thread two grabs that resource and 
     completes the update.  Then thread one gets to complete the update, 
     which over writes thread two's work.
     DEMO:  1) Run as is - see missing counts from race condition, Run severa times, values change  
            2) Uncomment "synchronized(countLock){ }" - see counts work
            Synchronized creates a lock on that block of code, no other threads can 
            execute code within a block that another thread has a lock.
        3) Comment ArrayList, unComment Vector - See no loss in collection
            Vectors work like ArrayList, but Vectors are "Thread Safe"
         May use this code as long as attribution to the author remains intact.
     /mf
*/ 

public class RaceCondition {
    private ArrayList<Integer> raceList = new ArrayList<Integer>(); // simple add(#)
//  private Vector<Integer> raceList = new Vector<Integer>(); // simple add(#)

    private String countLock="lock";    // Object use for locking the raceCount
    private int raceCount = 0;        // simple add 1 to this counter
    private int MAX = 10000;        // Do this 10,000 times
    private int NUM_THREADS = 100;    // Create 100 threads

    public static void main(String [] args) {
    new RaceCondition();
    }

    public RaceCondition() {
    ArrayList<Thread> arT = new ArrayList<Thread>();

    // Create thread objects, add them to an array list
    for( int i=0; i<NUM_THREADS; i++){
        Thread rt = new RaceThread( ); // i );
        arT.add( rt );
    }

    // Start all object at once.
    for( Thread rt : arT ){
        rt.start();
    }

    // Wait for all threads to finish before we can print totals created by threads
    for( int i=0; i<NUM_THREADS; i++){
        try { arT.get(i).join(); }
        catch( InterruptedException ie ) { System.out.println("Interrupted thread "+i); }
    }

    // All threads finished, print the summary information.
    // (Try to print this informaiton without the join loop above)
    System.out.printf("\nRace condition, should have %,d. Really have %,d in array, and count of %,d.\n",
                MAX*NUM_THREADS, raceList.size(), raceCount );
    System.out.printf("Array lost %,d. Count lost %,d\n",
             MAX*NUM_THREADS-raceList.size(), MAX*NUM_THREADS-raceCount );
    }   // end RaceCondition constructor



    class RaceThread extends Thread {
    public void run() {
        for ( int i=0; i<MAX; i++){
        try {
            update( i );        
        }    // These  catches show when one thread steps on another's values
        catch( ArrayIndexOutOfBoundsException ai ){ System.out.print("A"); }
        catch( OutOfMemoryError oome ) { System.out.print("O"); }
        }
    }

    // so we don't lose counts, need to synchronize on some object, not primitive
    // Created "countLock" to show how this can work.
    // Comment out the synchronized and ending {, see that we lose counts.

//    public synchronized void update(int i){   // use A
    public void update(int i){                  // remove this when adding A
//      synchronized(countLock){            // or B
//      synchronized(this){             // or C
        raceCount = raceCount + 1;
        raceList.add( i );      // use Vector  
//          }           // end block for B or C
    }   // end update

    }   // end RaceThread inner class


} // end RaceCondition outter class

同步的方法

优点:

您的IDE可以指示同步方法。 语法更加紧凑。 强制将同步块分割为单独的方法。

缺点:

与此同步，因此外部人员也可以与之同步。 将代码移到同步块之外更加困难。

同步块

优点:

允许为锁使用私有变量，从而将锁强制留在类内部。 同步块可以通过搜索变量的引用来找到。

缺点:

语法更复杂，因此使代码更难阅读。

就我个人而言，我更喜欢使用同步方法，类只关注需要同步的东西。这样的类应该尽可能小，所以应该很容易检查同步。其他人不需要关心同步。

关于使用同步块的重要提示:小心你使用的锁对象!

上面user2277816的代码片段说明了这一点，将字符串字面值的引用用作锁定对象。 意识到字符串字面值在Java中是自动互缩的，您应该开始看到问题所在:在字面值“锁”上同步的每段代码都共享同一个锁!这很容易导致完全不相关的代码段发生死锁。

您需要注意的不仅仅是String对象。装箱的原语也是一种危险，因为autoboxing和valueOf方法可以重用相同的对象，这取决于值。

有关更多信息，请参阅: https://www.securecoding.cert.org/confluence/display/java/LCK01-J.+Do+not+synchronize+on+objects+that+may+be+reused

注意:静态同步方法和块工作在Class对象上。

public class MyClass {
   // locks MyClass.class
   public static synchronized void foo() {
// do something
   }

   // similar
   public static void foo() {
      synchronized(MyClass.class) {
// do something
      }
   }
}

唯一真正的区别是同步块可以选择在哪个对象上同步。同步方法只能使用'this'(或同步类方法的相应Class实例)。例如，它们在语义上是等价的:

synchronized void foo() {
  ...
}

void foo() {
    synchronized (this) {
      ...
    }
}

后者更灵活，因为它可以竞争任何对象(通常是成员变量)的关联锁。它也更细粒度，因为您可以在块之前和块之后执行并发代码，但仍然在方法中。当然，您也可以通过将并发代码重构为单独的非同步方法来轻松地使用同步方法。使用任何使代码更容易理解的方法。

在实际应用中，同步方法相对于同步块的优势在于它们更能抵抗白痴;因为您不能选择任意对象来锁定，所以您不能滥用synchronized方法语法来做一些愚蠢的事情，比如锁定字符串文字或锁定从线程下面更改的可变字段的内容。

另一方面，使用同步方法，您无法保护锁不被任何可以获得对象引用的线程获取。

因此，在方法上使用synchronized作为修饰符可以更好地保护你的奶牛免受伤害，而将synchronized块与私有final锁对象结合使用则可以更好地保护你自己的代码免受奶牛的伤害。