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 of the Java Language specification \[[JLS 05|AA. Java References#JLS 05]\] describes how method synchronization works:

{quote}
A class method that is declared {{synchronized}} synchronizes on the lock associated with the {{Class}} object of the class.
{quote}

This does not mean that a subclass using {{getClass()}} can only synchronize on the {{Class}} object of the base class. In fact, it will lock on its own {{Class}} object, which may or may not be what the programmer had in mind. The intent should be clearly documented or annotated. Note that if a subclass does not override an accessible noncompliant superclass's method, it inherits the method which may lead to the false conclusion that the superclass's intrinsic lock is available in the subclass.

When synchronizing on a class literal, the corresponding lock object should not be accessible to untrusted code. If the class is package-private, callers from other packages may not access the class object, ensuring its trustworthiness as an intrinsic lock object. For more information, see [CON04-J. Use private final lock objects to synchronize classes that may interact with untrusted code].


h2. Noncompliant Code Example ({{getClass()}} lock object)

In this noncompliant code example, the {{parse()}} method of class {{Base}} parses a date and synchronizes on the class object returned by {{getClass()}}. The class {{Derived}} also inherits the {{parse()}} method. However, this inherited method synchronizes on the class object of {{Derived}} because of the particular return value of {{getClass()}}. 

{{Derived}} also adds a method {{doSomethingAndParse()}} that locks on the class object of the base class because the developer misconstrued that the {{parse()}} method in {{Base}} always obtains a lock on the {{Base}} class object, and {{doSomethingAndParse()}} must follow the same locking policy. Consequently, the class {{Derived}} has two different locking strategies and is not thread-safe.

{code:bgColor=#FFcccc}
class Base {
  static DateFormat FORMATformat =
      DateFormat.getDateInstance(DateFormat.MEDIUM);
	  
  public Date parse(String str) throws ParseException {
    synchronized (getClass()) {
      return FORMATformat.parse(str);
    }
  }
}

class Derived extends Base {
  public Date doSomethingAndParse(String str) throws ParseException {	  
    synchronized (Base.class) {
      // ...
      return FORMATformat.parse(str); 
    }
  }
}
{code}

{mc}
// Hidden main() method to be put in class Derived
// Prints arbitrary date values and throws exceptions at times

public static void main(String[] args) {
  for(int i = 0; i < 100; i++) {
    new Thread(new Runnable() {
      @Override public void run() {	    	 
        try {
  	 System.out.println(new Derived().parse("Jan 1, 2010"));
	} catch (ParseException e) {
	  // Forward to handler
	}
      }
    }).start();
        
    new Thread(new Runnable() {
      @Override public void run() {
         try {
    	   System.out.println(new Derived().doSomethingAndParse("Jan 2, 2010"));
	 } catch (ParseException e) {
	  // Forward to handler
	 }
      }
    }).start();	   
  }  
}
{mc}


h2. Compliant Solution (class name qualification)

Explicitly define the name of the class through name qualification ({{Base}} in this compliant solution) in the synchronized block. 

{code:bgColor=#ccccff}

class Base {
  static DateFormat FORMATformat =
      DateFormat.getDateInstance(DateFormat.MEDIUM);
	  
  public Date parse(String str) throws ParseException {
    synchronized (Base.class) {
      return FORMATformat.parse(str);
    }
  }
}

// ...
{code}

 not expose standard APIs that use the immediate caller's class loader instance to untrusted code] for more information.)


h2. Noncompliant Code Example ({{getClass()}} lock object, inner class)

This noncompliant code example follows from the previous one in that it synchronizes on the class object returned by {{getClass()}} in the {{parse()}} method of class {{Base}}. It also uses a nested class {{Helper}}, whose {{doSomethingAndParse()}} method incorrectly synchronizes by invoking {{getClass()}}. 

{code:bgColor=#FFcccc}
class Base {
  static DateFormat FORMATformat =
      DateFormat.getDateInstance(DateFormat.MEDIUM);
	  
  public Date parse(String str) throws ParseException {
    synchronized (getClass()) { // Intend to synchronizes on Base.class) {
      return FORMATformat.parse(str);
    }
  }

  public Date doSomething(String str) throws ParseException {
    return new Helper().doSomethingAndParse(str);
  }
  
  private class Helper {
    public Date doSomethingAndParse(String str) throws ParseException {	  
      synchronized (getClass()) { // Synchronizes on getClass()Helper.class
        // ...
        return FORMATformat.parse(str); 
      }
    }
  }
}
{code}

h2. Compliant Solution (class name qualification)

This compliant solution synchronizes using a {{Base}} class literal in the {{parse()}} and {{doSomethingAndParse()}} methods.

{code:bgColor=#ccccff}
class Base {
  // ...

  public Date parse(String str) throws ParseException {
    synchronized (Base.class) {
      return FORMATformat.parse(str);
    }
  }

  private class Helper {
    public Date doSomethingAndParse(String str) throws ParseException {	  
      synchronized (Base.class) { // Synchronizes on Base class literal
        // ...
        return FORMATformat.parse(str); 
      }
    }
  }
}
{code}