Page History

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Synchronizing

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on

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the

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return

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value

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of

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the

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Object.getClass()

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method

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can

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lead

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to

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unexpected

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behavior.

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Whenever

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the

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implementing

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class

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is

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subclassed,

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the

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subclass

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locks

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on

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the

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subclass's

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type. The Class object of the subclass is entirely distinct from the Class object of the parent class.

According to The Java Language Specification, §4.3.2

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, "The

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Class

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Object" [JLS 2005]:

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

Programmers who interpret this to mean that a subclass using getClass() will synchronize on the Class object of the base class are incorrect. The subclass will actually lock on its own Class object, which may or may not be what the programmer intended. Consequently, programs must not synchronize on the class object returned by getClass().

The programmer's actual intent should be clearly documented or annotated. Note that when a subclass fails to 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 be inaccessible to untrusted code. Callers from other packages cannot access class objects that are package-private; consequently, synchronizing on the intrinsic lock object of such classes is permitted (see LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code for more information).

Noncompliant Code Example (getClass() Lock Object)

In this noncompliant code example, the parse() method of the Base class parses a date and synchronizes on the class object returned by getClass(). The Derived class also inherits the parse() method. However, this inherited method synchronizes on Derived's class object because the inherited parse method's invocation of getClass() is really an invocation of this.getClass(), and the this argument is a reference to the instance of the Derived class.

The Derived class also adds a doSomethingAndParse() method 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 Derived class has two different locking strategies and fails to be thread-safe.

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

{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 intended. 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 format =
      DateFormat.getDateInstance(DateFormat.MEDIUM);
	  
  public Date parse(String str) throws ParseException {
    synchronized (getClass()) {
      return format.parse(str);
    }
  }
}

class Derived extends Base {
  public Date doSomethingAndParse(String str) throws ParseException {	  
    synchronized (Base.class) {
      // ...
      return format.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)

In this compliant solution, the class name providing the lock ({{Base}}) is fully qualified. 

{code:bgColor=#ccccff}

Compliant Solution (Class Name Qualification)

In this compliant solution, the class name providing the lock (Base) is fully qualified:

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

// ...
{code}

This

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code

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example

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always

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synchronizes

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on

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the

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Base.class

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object,

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even

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when it

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is

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called

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from

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a

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Derived

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object.

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Compliant Solution (Class.forName()

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)

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This

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compliant

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solution

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uses

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the

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Class.forName()

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method

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to

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synchronize

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on

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the

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Base

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class's

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Class object:

Code Block
bgColor #ccccff
}} object. {code:bgColor=#ccccff} class Base { static DateFormat format = DateFormat.getDateInstance(DateFormat.MEDIUM); public Date parse(String str) throws ParseException { try { synchronized (Class.forName("Base")) { return format.parse(str); } } } // ... {code} It is important that untrusted inputs are not accepted as arguments while loading classes using {{Class.forName()}}. See [SEC05-J. Do not expose standard APIs that use the immediate caller's class loader instance to untrusted code] for more information. h2. Noncompliant Code Example ({{getClass catch (ClassNotFoundException x) { // "Base" not found; handle error } return null; } } // ...

Never accept untrusted inputs as arguments while loading classes using Class.forName() (see SEC03-J. Do not load trusted classes after allowing untrusted code to load arbitrary classes for more information).

Noncompliant Code Example (getClass() Lock Object, Inner Class)

This noncompliant code example synchronizes on the class object returned by getClass() in the parse() method of class Base. The Base class also has a nested Helper class whose doSomethingAndParse() method incorrectly synchronizes on the value returned by getClass().

()}} Lock Object, Inner Class)

This noncompliant code example synchronizes on the class object returned by {{getClass()}} in the {{parse()}} method of {{Base}} class. The {{Base}} class also has a nested {{Helper}} class, whose {{doSomethingAndParse()}} method incorrectly synchronizes on the value returned by {{getClass()}}. 

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

The

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call

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to

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getClass()

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in

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the Helper class returns a Helper class object instead of the Base class object. Consequently, a thread that calls Base.parse()

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locks

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on

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a

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different

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object

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than

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a

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thread

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that

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calls

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Base.doSomething()

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.

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It

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is

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easy

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to

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overlook

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concurrency

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errors

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in

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inner

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classes

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because

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they

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exist

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within

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the

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body

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of

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the

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containing

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outer

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class.

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A

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reviewer

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might

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incorrectly

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assume

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that

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the

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two

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classes

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have

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the

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same

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locking

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strategy.

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Compliant

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Solution

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(Class

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Name

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Qualification)

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This

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compliant

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solution

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synchronizes

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using

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a

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Base

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class

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literal

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in

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the

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parse()

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and

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doSomethingAndParse() methods:

Code Block
bgColor #ccccff
}} methods. {code:bgColor=#ccccff} class Base { // ... public Date parse(String str) throws ParseException { synchronized (Base.class) { return format.parse(str); } } private class Helper { public Date doSomethingAndParse(String str) throws ParseException { synchronized (Base.class) { // Synchronizes on Base class literal // ... return format.parse(str); } } } } {code}

Consequently,

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both

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Base

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and

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Helper

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lock

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on

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Base

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's

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intrinsic

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lock.

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Similarly,

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the

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Class.

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forName()

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method

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can

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be

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used

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instead

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of

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a

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class

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literal.

Risk Assessment

Synchronizing on the class object returned by getClass() can result in nondeterministic behavior.

Automated Detection

Some static analysis tools can detect violations of this rule

Bibliography

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