我想这个问题是关于线程安全单例和带有双重检查锁定的惰性初始化之间的区别。当我需要实现某些特定的单例时，我总是会参考这篇文章。

这是一个线程安全单例:

// Java program to create Thread Safe 
// Singleton class 
public class GFG  
{ 
  // private instance, so that it can be 
  // accessed by only by getInstance() method 
  private static GFG instance; 

  private GFG()  
  { 
    // private constructor 
  } 

 //synchronized method to control simultaneous access 
  synchronized public static GFG getInstance()  
  { 
    if (instance == null)  
    { 
      // if instance is null, initialize 
      instance = new GFG(); 
    } 
    return instance; 
  } 
} 

优点: 延迟初始化是可能的。 它是线程安全的。 缺点: getInstance()方法是同步的，因此它会导致性能变慢，因为多个线程不能同时访问它。

这是一个带有双重检查锁定的Lazy初始化:

// Java code to explain double check locking 
public class GFG  
{ 
  // private instance, so that it can be 
  // accessed by only by getInstance() method 
  private static GFG instance; 

  private GFG()  
  { 
    // private constructor 
  } 

  public static GFG getInstance() 
  { 
    if (instance == null)  
    { 
      //synchronized block to remove overhead 
      synchronized (GFG.class) 
      { 
        if(instance==null) 
        { 
          // if instance is null, initialize 
          instance = new GFG(); 
        } 

      } 
    } 
    return instance; 
  } 
} 

优点: 延迟初始化是可能的。 它也是线程安全的。 克服了synchronized关键字导致的性能下降。 缺点: 第一次，它会影响性能。 由于双止回锁方法的缺点是可以承受的，所以可以 用于高性能多线程应用程序。

详情请参考这篇文章:

https://www.geeksforgeeks.org/java-singleton-design-pattern-practices-examples/

使用同步块，您可以有多个同步器，因此多个同时但不冲突的事情可以同时进行。

As already said here synchronized block can use user-defined variable as lock object, when synchronized function uses only "this". And of course you can manipulate with areas of your function which should be synchronized. But everyone says that no difference between synchronized function and block which covers whole function using "this" as lock object. That is not true, difference is in byte code which will be generated in both situations. In case of synchronized block usage should be allocated local variable which holds reference to "this". And as result we will have a little bit larger size for function (not relevant if you have only few number of functions).

你可以在这里找到更详细的解释: http://www.artima.com/insidejvm/ed2/threadsynchP.html

主要的区别是，如果你使用同步块，你可以锁定一个对象，而不是这个，这允许更灵活。

假设您有一个消息队列和多个消息生产者和消费者。我们不希望生产者相互干扰，但是消费者应该能够检索消息，而不必等待生产者。 我们只需要创建一个对象

Object writeLock = new Object();

从现在开始，每当制作人想要添加一条新信息时，我们就会锁定它:

synchronized(writeLock){
  // do something
}

因此，消费者可能仍会阅读，而生产者将被锁定。

TLDR;不要使用synchronized修饰符或synchronized(this){…}表达式but synchronized(myLock){…其中myLock是一个持有私有对象的最终实例字段。

在方法声明中使用synchronized修饰符与在方法主体中使用synchronized(..){}表达式的区别如下:

The synchronized modifier specified on the method's signature is visible in the generated JavaDoc, is programmatically determinable via reflection when testing a method's modifier for Modifier.SYNCHRONIZED, requires less typing and indention compared to synchronized(this) { .... }, and (depending on your IDE) is visible in the class outline and code completion, uses the this object as lock when declared on non-static method or the enclosing class when declared on a static method. The synchronized(...){...} expression allows you to only synchronize the execution of parts of a method's body, to be used within a constructor or a (static) initialization block, to choose the lock object which controls the synchronized access.

然而，使用synchronized修饰符或synchronized(…){…}使用this作为锁对象(如synchronized(this){…})，也有同样的缺点。两者都使用它自己的实例作为锁对象进行同步。这是很危险的，因为不仅对象本身，而且任何其他持有该对象引用的外部对象/代码也可以将其用作同步锁，这可能会产生严重的副作用(性能下降和死锁)。

因此，最佳实践是既不使用synchronized修饰符，也不使用synchronized(…)表达式作为锁对象，而是使用该对象的私有锁对象。例如:

public class MyService {
    private final lock = new Object();

    public void doThis() {
       synchronized(lock) {
          // do code that requires synchronous execution
        }
    }

    public void doThat() {
       synchronized(lock) {
          // do code that requires synchronous execution
        }
    }
}

您也可以使用多个锁对象，但是需要特别注意，以确保在嵌套使用时不会导致死锁。

public class MyService {
    private final lock1 = new Object();
    private final lock2 = new Object();

    public void doThis() {
       synchronized(lock1) {
          synchronized(lock2) {
              // code here is guaranteed not to be executes at the same time
              // as the synchronized code in doThat() and doMore().
          }
    }

    public void doThat() {
       synchronized(lock1) {
              // code here is guaranteed not to be executes at the same time
              // as the synchronized code in doThis().
              // doMore() may execute concurrently
        }
    }

    public void doMore() {
       synchronized(lock2) {
              // code here is guaranteed not to be executes at the same time
              // as the synchronized code in doThis().
              // doThat() may execute concurrently
        }
    }
}

通常在方法级别上使用锁是不礼貌的。为什么要通过锁定整个方法来锁定一段不能访问任何共享资源的代码呢?因为每个对象都有一个锁，所以可以创建虚拟对象来实现块级同步。 块级的效率更高，因为它不锁定整个方法。

这里有一些例子

方法级

class MethodLevel {

  //shared among threads
SharedResource x, y ;

public void synchronized method1() {
   //multiple threads can't access
}
public void synchronized method2() {
  //multiple threads can't access
}

 public void method3() {
  //not synchronized
  //multiple threads can access
 }
}

块级别

class BlockLevel {
  //shared among threads
  SharedResource x, y ;

  //dummy objects for locking
  Object xLock = new Object();
  Object yLock = new Object();

    public void method1() {
     synchronized(xLock){
    //access x here. thread safe
    }

    //do something here but don't use SharedResource x, y
    // because will not be thread-safe
     synchronized(xLock) {
       synchronized(yLock) {
      //access x,y here. thread safe
      }
     }

     //do something here but don't use SharedResource x, y
     //because will not be thread-safe
    }//end of method1
 }

(编辑)

对于像Vector和Hashtable这样的集合，当ArrayList或HashMap不同步时，它们是同步的，你需要设置synchronized关键字或调用Collections synchronized方法:

Map myMap = Collections.synchronizedMap (myMap); // single lock for the entire map
List myList = Collections.synchronizedList (myList); // single lock for the entire list