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Wiki MarkupAccording to the The Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\], section 15, §15.8.3 {{this}}, "this" [JLS 2015]:

When used as a primary expression, the keyword this denotes a value that is a reference to the object for which the instance method was invoked (§15§15.12), or to the object being constructed....

The type of this is the class C or interface type T within which the keyword this occurs....

At run time, the class of the actual object referred to may be the class C or any subclass of C T, if T is a class type, or a class that is a subtype of T.

The this reference is said to have escaped when it is made available beyond its current scope. Common Following are common ways by which the this reference can escape include:

• Returning this from a nonprivate

  method
• Passing this as an argument to an alien method
• Publishing this

  , overridable method that is invoked from the constructor of

  an object under construction
• Publishing this such that pieces of code beyond its current scope can obtain its reference
• Calling a non-final method from a constructor (MET04

  a class whose object is being constructed (for more information, see MET05-J. Ensure that constructors do not call overridable methods)

• Overriding the finalizer of a non-final class and obtaining the this reference of a partially constructed instance, when the construction of the object ceases (OBJ04-J. Do not allow partially initialized objects to be accessed)
• Passing internal object state to an alien method and consequently, exposing the this reference of internal objects

This guideline focuses on the potential consequences of this escaping during object construction including race conditions and improper initialization. For example, declaring a field final ensures that all threads will see it fully initialized only when the this reference does not escape during the corresponding object's construction. The guideline CON26-J. Do not publish partially-constructed objects discusses the guarantees provided by various mechanisms for safe publication and relies on conformance to this guideline. In general, it is important to detect cases where the this reference can leak out beyond the scope of the current context. In particular, public variables and methods should be carefully scrutinized.

Noncompliant code Example (publish before initialization)

This noncompliant code example publishes the this reference before initialization has concluded, by storing it in a public static volatile class field. Consequently, other threads may obtain a partially initialized Publisher instance.

class Publisher {
  public static volatile Publisher pub; // Also noncompliant if field is nonvolatile
  int num;

  Publisher(int number) {
    pub = this; 
    // Initialization 
    this.num = number;
    // ...
  }
}

Also, if the object initialization (and consequently, its construction) depends on a security check within the constructor, the security check will be bypassed if an untrusted caller obtains the partially initialized instance (see OBJ04-J. Do not allow partially initialized objects to be accessed for more details).

Noncompliant Code Example (nonvolatile public static field)

This noncompliant code example publishes the this reference in the last statement of the constructor but is still vulnerable because the field pub is not declared volatile and has public accessibility.

• .

• Returning this from a nonprivate method of a mutable class, which allows the caller to manipulate the object's state indirectly. This situation commonly occurs in method-chaining implementations (see VNA04-J. Ensure that calls to chained methods are atomic for more information).
• Passing this as an argument to an alien method invoked from the constructor of a class whose object is being constructed.
• Using inner classes. An inner class implicitly holds a reference to the instance of its outer class unless the inner class is declared static.
• Publishing by assigning this to a public static variable from the constructor of a class whose object is being constructed.
• Throwing an exception from a constructor. Doing so may cause code to be vulnerable to a finalizer attack (see OBJ11-J. Be wary of letting constructors throw exceptions for more information).
• Passing internal object state to an alien method, which enables the method to retrieve the this reference of the internal member object.

This rule describes the potential consequences of allowing the this reference to escape during object construction, including race conditions and improper initialization. For example, declaring a field final ordinarily ensures that all threads see the field in a fully initialized state; however, allowing the this reference to escape during object construction can expose the field to other threads in an uninitialized or partially initialized state. TSM03-J. Do not publish partially initialized objects, which describes the guarantees provided by various mechanisms for safe publication, relies on conformance to this rule. Consequently, programs must not allow the this reference to escape during object construction.

In general, it is important to detect cases in which the this reference can leak out beyond the scope of the current context. In particular, public variables and methods should be carefully scrutinized.

Noncompliant Code Example (Publish before Initialization)

This noncompliant code example publishes the this reference before initialization has concluded by storing it in a public static volatile class field. Consequently, other threads can obtain a partially initialized Publisher instance.

final class Publisher {
  public static volatile Publisher published;
 

class Publisher {
  public static Publisher pub;
  int num;

  Publisher(int number) {
    //published Initialization= this;
    this// Initialization
    this.num = number;
    // ...
    pub = this;
  }
}

Because, the field is nonvolatile and nonfinal, the statements within the constructor can be reordered by the compiler in such a way that the this reference is published before the initialization statements are executedIf an object's initialization (and consequently, its construction) depends on a security check within the constructor, the security check can be bypassed when an untrusted caller obtains the partially initialized instance (see OBJ11-J. Be wary of letting constructors throw exceptions for more information).

Noncompliant Code Example (

...

Nonvolatile Public Static Field)

This noncompliant code example declares the pub field as volatile and reduces the accessibility of the static class field to package-private so that untrusted callers beyond the current package cannot obtain the this instancepublishes the this reference in the last statement of the constructor. It remains vulnerable because the published field has public accessibility and the programmer has failed to declare it as volatile.

final class Publisher {
  public static volatile Publisher pubpublished;
  int num;

  Publisher(int number) {
    // Initialization 
    this.num = number;
    // ...
    pubpublished = this;
  }
}

The constructor publishes the this reference after initialization has concluded. However, it is unsuitable because a legit caller can see the default value of the field, before it is initialized (a violation of CON26-J. Do not publish partially-constructed objects).

If the pub field is not declared as volatile initialization statements may be reordered. The Java compiler does not allow declaring the static pub field as final in this case.

...

Because the field is nonvolatile and nonfinal, the statements within the constructor can be reordered by the compiler in such a way that the this reference is published before the initialization statements have executed.

Compliant Solution (Volatile Field and Publish after Initialization)

This compliant solution removes both declares the internal published field and provides a newInstance() factory method which can be used to obtain an instance of the Publisher object. This ensures that threads do not see a compromised Publisher instancevolatile and reduces its accessibility to package-private so that callers outside the current package scope cannot obtain the this reference.

final class Publisher {
  static volatile Publisher published;
 final int num;

  private Publisher(int number) {
    // Initialization 
    this.num = number;
  }
  
  public static Publisher newInstance(int number) {// ...
    Publisher pupublished = new Publisher(number);  
    return puthis;
  }
}

Noncompliant Code Example (handlers)

This noncompliant code example defines the ExceptionReporter interface that is implemented by the class DefaultExceptionReporter. This class is useful for reporting exceptions after filtering out any sensitive information (EXC01-J. Use a class dedicated to reporting exceptions). The DefaultExceptionReporter constructor, prematurely publishes the this reference before construction of the object has concluded. This occurs in the last statement of the constructor (er.setExceptionReporter(this)) which sets the exception reporter. Because it is the last statement of the constructor, this may be misconstrued as benign.

The constructor publishes the this reference after initialization has concluded. However, the caller that instantiates Publisher must ensure that it cannot see the default value of the num field before it is initialized; to do otherwise would violate TSM03-J. Do not publish partially initialized objects. Consequently, the field that holds the reference to Publisher might need to be declared volatile in the caller.

Initialization statements may be reordered when the published field is not declared volatile. The Java compiler, however, forbids declaring fields as both volatile and final.

The class Publisher must also be final; otherwise, a subclass can call its constructor and publish the this reference before the subclass's initialization has concluded.

Compliant Solution (Public Static Factory Method)

This compliant solution eliminates the internal member field and provides a newInstance() factory method that creates and returns a Publisher instance:

final class Publisher {
  final int num;

  private Publisher(int number

// Interface ExceptionReporter
public interface ExceptionReporter {
  public void setExceptionReporter(ExceptionReporter er);
  public void report(Throwable exception);
}

// Class DefaultExceptionReporter
public class DefaultExceptionReporter implements ExceptionReporter {
  public DefaultExceptionReporter(ExceptionReporter er) {
    // Carry out initialization 
 Initialization
   // Incorrectly publishes the "this".num reference
    er.setExceptionReporter(this)= number;
  }

  public finalstatic voidPublisher setExceptionReporternewInstance(ExceptionReporterint ernumber) { 
    //Publisher Setspublished the= reporternew Publisher(number);
  }

  public void report(Throwable exception) { /* default implementation */ }
}

// Class MyExceptionReporter derives from DefaultExceptionReporter
public class MyExceptionReporter extends DefaultExceptionReporterreturn published;
  }
}

This approach ensures that threads cannot see an inconsistent Publisher instance. The num field is also declared final, making the class immutable and consequently eliminating the possibility of obtaining a partially initialized object.

Noncompliant Code Example (Handlers)

This noncompliant code example defines the ExceptionReporter interface:

public interface ExceptionReporter {
  privatepublic finalvoid Logger loggersetExceptionReporter(ExceptionReporter er);
  
public void public MyExceptionReporterreport(ExceptionReporter er) {
    super(er); // Calls superclass's constructor
    logger = Logger.getLogger("com.organization.Log"Throwable exception);
  }

  public void report(Throwable t) {
    logger.log(Level.FINEST,"Loggable exception occurred",t);
  }
}

The class MyExceptionReporter subclasses DefaultExceptionReporter with the intent of adding a logging mechanism that logs critical messages before an exception is reported. Its constructor invokes the superclass DefaultExceptionReporter's constructor (a mandatory first step) which publishes the exception reporter before the initialization of the subclass has concluded. Note that the subclass initialization consists of obtaining an instance of the default logger. Publishing the exception reporter is equivalent to setting it so that it can receive and handle exceptions from that point onwards.

If any exception occurs before the call to Logger.getLogger() in the subclass, it is not logged. Instead, a NullPointerException is generated which may itself be consumed by the reporting mechanism.

This erroneous behavior results from the race condition between an oncoming exception and the initialization of the subclass. If the exception comes too soon, it finds the subclass in a compromised state. This behavior is especially counter intuitive because logger is declared final and is not expected to contain an unintialized value.

This issue can also occur when a listener is prematurely published. Consequently, it will start receiving event notifications even before the subclass's initialization has concluded.

Compliant Solution

This compliant solution defines a method setReporter() in class MyExceptionReporter. The method explicitly calls the superclass's setExceptionReporter() method, publishing a reference to its own Class object. It is not permissible to publish the reference in the constructor for MyExceptionReporter for reasons noted earlier in the noncompliant code example.

}

This interface is implemented by the DefaultExceptionReporter class, which reports exceptions after filtering out any sensitive information (see ERR00-J. Do not suppress or ignore checked exceptions for more information).

The DefaultExceptionReporter constructor prematurely publishes the this reference before construction of the object has concluded. This occurs in the last statement of the constructor (er.setExceptionReporter(this)), which sets the exception reporter. Because it is the last statement of the constructor, this may be misconstrued as benign.

// Class DefaultExceptionReporter
public class DefaultExceptionReporter implements ExceptionReporter {
  public DefaultExceptionReporter(ExceptionReporter er) {
    // Carry out initialization
    // Incorrectly publishes the "this" reference
    er.setExceptionReporter(this);
  }

  // Implementation of setExceptionReporter() and report()
}

The MyExceptionReporter class subclasses DefaultExceptionReporter with the intent of adding a logging mechanism that logs critical messages before reporting an exception.

// Class MyExceptionReporter derives from DefaultExceptionReporter
public class MyExceptionReporter extends DefaultExceptionReporter {
  private final Logger logger;

  public MyExceptionReporter

public class MyExceptionReporter extends DefaultExceptionReporter {
  // ...
  public void setReporter(ExceptionReporter er) {
    super.setExceptionReporter(thiser); // Calls superclass's constructor
    // Obtain the }
}

Because, setReporter() is an instance method, it cannot be invoked until the subclass's initialization has finished.

Noncompliant Code Example (inner class)

It is possible for the {{this}} reference to implicitly get leaked outside the scope \[[Goetz 02|AA. Java References#Goetz 02]\]. Consider inner classes that maintain a copy of the {{this}} reference of the outer object. In this noncompliant code example, the constructor for class {{BadExceptionReporter}} uses an anonymous inner class to publish a {{filter()}} method. 

public class BadExceptionReporter implements ExceptionReporter {
  public BadExceptionReporter(ExceptionReporter er) { 
    er.setExceptionReporter(new DefaultExceptionReporter(er) {
    public void report(Throwable t) {
      filter(t);
    }		
  });
}
  public void filter(Throwable t) { 
    // Filters sensitive exceptions 
  }

  public void setExceptionReporter(ExceptionReporter er) { 
    // Sets the reporter 
  }

  public void report(Throwable exception) { 
    // Default implementation 
  }

}

The problem occurs because the this reference of the outer class is published by the inner class so that other threads can see it. If the class is subclassed, the issue described in the previous noncompliant code example resurfaces.

Compliant Solution

A {{private}} constructor alongside a {{public}} factory method can be used to safely publish the {{filter()}} method from within the constructor \[[Goetz 06|AA. Java References#Goetz 06]\].

public class GoodExceptionReporter implements ExceptionReporter {
  private final DefaultExceptionReporter er;

  private GoodExceptionReporter(ExceptionReporter excr) {
    er = new DefaultExceptionReporter(excr) {
      public void report(Throwable t) {
        filter(t);
      } 
    };
  }
 
  public static GoodExceptionReporter newInstance(ExceptionReporter excr) {
    GoodExceptionReporter ger = new GoodExceptionReporter(excr);
    excr.setExceptionReporter(ger.er);
    return ger;
  }

  public void setExceptionReporter(ExceptionReporter er) { }

  public void filter(Throwable t) { }

  public void report(Throwable exception) { }
}

Noncompliant Code Example (thread)

This noncompliant code example starts a thread from within the constructor.

public someConstructor() {
  thread = new MyThread(this);
  thread.start();
}

This allows the new thread to access the {{this}} reference of the current object \[[Goetz 02|AA. Java References#Goetz 02], [Goetz 06|AA. Java References#Goetz 06]\].

Compliant Solution

In this compliant solution, even though the thread is created in the constructor, it is not started until its {{start()}} method is called from method {{startThread()}} \[[Goetz 02|AA. Java References#Goetz 02], [Goetz 06|AA. Java References#Goetz 06]\].

public someConstructor() {
  thread = new MyThread(this);
}

public void startThread() {
  thread.start();
}

Risk Assessment

Allowing the this reference to escape may result in improper initialization and runtime exceptions.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[JLS 05|AA. Java References#JLS 05]\] Keyword "this"
\[[Goetz 02|AA. Java References#Goetz 02]\]
\[[Goetz 06|AA. Java References#Goetz 06]\] 3.2. Publication and Escape

default logger
    logger = Logger.getLogger("com.organization.Log");
  }

  public void report(Throwable t) {
    logger.log(Level.FINEST,"Loggable exception occurred", t);
  }
}

The MyExceptionReporter constructor invokes the DefaultExceptionReporter superclass's constructor (a mandatory first step), which publishes the exception reporter before the initialization of the subclass has concluded. Note that the subclass initialization consists of obtaining an instance of the default logger. Publishing the exception reporter is equivalent to setting it to receive and handle exceptions from that point on.

Logging will fail when an exception occurs before the call to Logger.getLogger() in the MyExceptionReporter subclass because dereferencing the uninitialized logger field generates a NullPointerException, which could itself be consumed by the reporting mechanism without being logged.

This erroneous behavior results from the race condition between an oncoming exception and the initialization of MyExceptionReporter. If the exception arrives too soon, it will find the MyExceptionReporter object in an inconsistent state. This behavior is especially counterintuitive because logger has been declared final, so observing an uninitialized value would be unexpected.

Premature publication of an event listener causes a similar problem; the listener can receive event notifications before the subclass's initialization has finished.

Compliant Solution

Rather than publishing the this reference from the DefaultExceptionReporter constructor, this compliant solution adds a publishExceptionReporter() method to DefaultExceptionReporter to permit setting the exception reporter. This method can be invoked on a subclass instance after the subclass's initialization has concluded.

public class DefaultExceptionReporter implements ExceptionReporter {
  public DefaultExceptionReporter(ExceptionReporter er) {
    // ...
  }

  // Should be called after subclass's initialization is over
  public void publishExceptionReporter() {
    setExceptionReporter(this); // Registers this exception reporter
  }

  // Implementation of setExceptionReporter() and report()
}

The MyExceptionReporter subclass inherits the publishExceptionReporter() method. Callers that instantiate MyExceptionReporter can use the resulting instance to set the exception reporter after initialization is complete.

// Class MyExceptionReporter derives from DefaultExceptionReporter
public class MyExceptionReporter extends DefaultExceptionReporter {
  private final Logger logger;

  public MyExceptionReporter(ExceptionReporter er) {
    super(er); // Calls superclass's constructor
    logger = Logger.getLogger("com.organization.Log");
  }
  // Implementations of publishExceptionReporter(), 
  // setExceptionReporter() and report()
  // are inherited
}

This approach ensures that the reporter cannot be set before the constructor has fully initialized the subclass and enabled logging.

Noncompliant Code Example (Inner Class)

Inner classes maintain a copy of the this reference of the outer object. Consequently, the this reference could leak outside the scope [Goetz 2002]. This noncompliant code example uses a different implementation of the DefaultExceptionReporter class. The constructor uses an anonymous inner class to publish an exception reporter.

public class DefaultExceptionReporter implements ExceptionReporter {
  public DefaultExceptionReporter(ExceptionReporter er) {
    er.setExceptionReporter(new ExceptionReporter() {
        public void report(Throwable t) {
          // report exception
        }
        public void setExceptionReporter(ExceptionReporter er) {
          // register ExceptionReporter
        }
    });
  }
  // Default implementations of setExceptionReporter() and report()
}

Other threads can see the this reference of the outer class because it is published by the inner class. Furthermore, the issue described in the noncompliant code example for handlers will resurface if the class is subclassed.

Compliant Solution

Use a private constructor and a public static factory method to safely publish the exception reporter from within the constructor [Goetz 2006a]:

public class DefaultExceptionReporter implements ExceptionReporter {
  private final ExceptionReporter defaultER;

  private DefaultExceptionReporter(ExceptionReporter excr) {
    defaultER = new ExceptionReporter() {
      public void report(Throwable t) {
        // Report exception
      }
      public void setExceptionReporter(ExceptionReporter er) {
        // Register ExceptionReporter
      }
    };
  }

  public static DefaultExceptionReporter newInstance(
                ExceptionReporter excr) {
    DefaultExceptionReporter der = new DefaultExceptionReporter(excr);
    excr.setExceptionReporter(der.defaultER);
    return der;
  }
  // Default implementations of setExceptionReporter() and report()
}

Because the constructor is private, untrusted code cannot create instances of the class; consequently, the this reference cannot escape. Using a public static factory method to create new instances also protects against untrusted manipulation of internal object state and publication of partially initialized objects (see TSM03-J. Do not publish partially initialized objects for additional information).

Noncompliant Code Example (Thread)

This noncompliant code example starts a thread inside the constructor:

final class ThreadStarter implements Runnable {
  public ThreadStarter() {
    Thread thread = new Thread(this);
    thread.start();
  }

  @Override public void run() {
    // ...
  }
}

The new thread can access the this reference of the current object [Goetz 2002], [Goetz 2006a]. Notably, the Thread() constructor is alien to the ThreadStarter class.

Compliant Solution (Thread)

This compliant solution creates and starts the thread in a method rather than in the constructor:

final class ThreadStarter implements Runnable {
  public void startThread() {
    Thread thread = new Thread(this);
    thread.start();
  }

  @Override public void run() {
    // ...
  }
}

Exceptions

TSM01-J-EX0: It is safe to create a thread in the constructor, provided the thread is not started until after object construction is complete, because a call to start() on a thread happens-before any actions in the started thread [JLS 2015].

Even though this code example creates a thread that references this in the constructor, the thread is started only when its start() method is called from the startThread() method [Goetz 2002], [Goetz 2006a].

final class ThreadStarter implements Runnable {
  Thread thread;

  public ThreadStarter() {
    thread = new Thread(this);
  }

  public void startThread() {
    thread.start();
  }

  @Override public void run() {
    // ...
  }
}

TSM01-J-EX1: Use of the ObjectPreserver pattern [Grand 2002] described in TSM02-J. Do not use background threads during class initialization is safe and is permitted.

Risk Assessment

Allowing the this reference to escape can result in improper initialization and runtime exceptions.

Automated Detection

Bibliography

Issue Tracking

||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||
|T|M|F|1270219843973|1270221864972|svoboda|"*Inner classes* implicitly hold a reference to the instance of the outer class, unless the inner class is declared as static." => Change inner classes to "An inner class implicitly holds ... "|
|T|M|F|1270220129871|1270755934790|rcs|"Note that this code also violates CON32-J. Protect accessible mutable static fields from untrusted code" => Not sure if I agree because the class is package-private and inaccessible to untrusted code|
|T|M|F|1270733657099|1271021912028|rcs_mgr|"A Runnable object's constructor may construct a Thread object around itself, as long as the thread is not actually started in the Runnable object's constructor." => I still think this info is redundant.|

...

Image Added Image Added Image AddedVOID CON14-J. Ensure atomicity of 64-bit operations      11. Concurrency (CON)      11. Concurrency (CON)