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Before the The garbage collector acts on an object to reclaim it, invokes object finalizer methods after it determines that the object is unreachable but before it reclaims the object's finalizer is executed. This is required to ensure that storage. Execution of the finalizer provides an opportunity to release resources such as open streams, files, and network connections are released as resource management does not happen automatically when memory is reclaimed. In Java, the finalize() method of java.lang.Object is used for this purpose.that might not otherwise be released automatically through the normal action of the garbage collector.

A sufficient number of problems are associated with finalizers to restrict their use to exceptional conditionsThere are a number of caveats associated with the use of finalizers:

• There is no fixed time for finalizers to get executed; this detail is JVM dependent: The only thing that is guaranteed is that if a finalizer gets executed, it will be before the garbage collector's second cycle. An object may become unreachable and yet its finalizer may not execute for an arbitrarily long time. No time-critical functionality should be implemented in the finalize() method. For instance, closing file handles is not recommended.at which finalizers must be executed because time of execution depends on the Java Virtual Machine (JVM). The only guarantee is that any finalizer method that executes will do so sometime after the associated object has become unreachable (detected during the first cycle of garbage collection) and sometime before the garbage collector reclaims the associated object's storage (during the garbage collector's second cycle). Execution of an object's finalizer may be delayed for an arbitrarily long time after the object becomes unreachable. Consequently, invoking time-critical functionality such as closing file handles in an object's finalize() method is problematic.

• The JVM may Do not depend on a finalizer for updating critical persistent state: It is possible for the JVM to terminate without invoking the finalizer on an unreachable object. Finalization on process termination is also not guaranteed to worksome or all unreachable objects. Consequently, attempts to update critical persistent state from finalizer methods can fail without warning. Similarly, Java lacks any guarantee that finalizers will execute on process termination. Methods such as System.gc(), System.runFinalization(), System.runFinalizersOnExit(), and Runtime.runFinalizersOnExit() are either just marginally better either lack such guarantees or have been deprecated because of lack of safety and potential for deadlock causing effects.

• Wiki MarkupAccording to the The Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] Section 12.6.2 "Finalizer Invocations are Not Ordered":

  Wiki Markup
  The Java programming language imposes no ordering on {{finalize}} method calls. Finalizers \[of different objects\] may be called in any order, or even concurrently.

  This can be a problem as slow running finalizers tend to block others in the queue.Specification (JLS), §12.6, "Finalization of Class Instances" [JLS 2015]:

  The Java programming language imposes no ordering on finalize() method calls. Finalizers [of different objects] may be called in any order, or even concurrently.

  One consequence is that slow-running finalizers can delay execution of other finalizers in the queue. Further, the lack of guaranteed ordering can lead to substantial difficulty in maintaining desired program invariants.

• Uncaught exceptions thrown during finalization are ignored. When an exception thrown in a finalizer propagates beyond the finalize() method, the process itself immediately stops and consequently fails to accomplish its sole purpose. This termination of the finalization process may or may not prevent all subsequent finalization from executing. The JLS fails to define this behavior, leaving it to the individual implementations.Effect of uncaught exceptions: An uncaught exception thrown during finalization is ignored. The finalization process itself stops immediately so it fails to accomplish its sole purpose.

• Coding errors that result in memory leaks can also cause finalizers to never execute to completionobjects to incorrectly remain reachable; consequently, their finalizers are never invoked.

• A programmer may can unintentionally resurrect the an object's reference in the finalize() method. While When this occurs, the garbage collector must determine yet again whether the object is free to be deallocated. Further, because the finalize() method is not invoked again.

• Superclasses that use finalizers bring additional burden to their extending classes. Consider an example from JDK 1.5 and earlier. The code snippet allocates a 16 MB buffer for backing a Swing Jframe. None of the JFrame APIs have a finalize() method, however, JFrame extends AWT Frame which has a finalize() method. The byte buffer continues to persist until its inherited finalize() method gets called, and persists for at least two garbage collection cycles.

Class MyFrame extends Jframe {
  private byte[] buffer = new byte[16 * 1024 * 1024]; // persists for at least two GC cycles 
}

• It is also imprudent to use finalizers for reclaiming scarce resources by enforcing garbage collection. Garbage collection usually depends on memory related traits and not on the scarcity of a particular resource. As a result, if memory is readily available, a scarce resource may get exhausted even in the presence of a finalizer. See FIO32-J. Ensure all resources are properly closed when they are no longer needed and CON02-J. Facilitate thread reuse by using Thread Pools for more details on handling resources correctly.

• A common myth is that finalizers aid garbage collection. On the contrary, they increase garbage collection time and introduce space overheads. They also fail to respect the modern generational garbage collectors. Another trap unfolds while trying to finalize reachable objects, an exercise that is always counterproductive.

• Wiki Markup

  It is not advisable to use any lock or sharing based mechanisms within a finalizer because of the inherent dangers of deadlock and starvation. On the other hand, it is easy to miss that there can be synchronization issues with the use of finalizers even if the source program is single-threaded. This is because the {{finalize()}} methods are called from their own threads (not from the {{main()}} thread). If a finalizer is necessary, the cleanup data structure should be protected from concurrent access (See \[[Boehm 05|AA. Java References#Boehm 05]\]).

Noncompliant Code Example

This noncompliant code example uses the System.runFinalizersOnExit() method to simulate a garbage collection run (note that this method is deprecated because of thread-safety issues MET36-J. Do not use deprecated or obsolete methods).

According to the Java API \[[API 06|AA. Java References#API 06]\] class {{System}}, {{runFinalizersOnExit()}} method documentation:

Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.

The class SubClass overrides the protected finalize method and performs cleanup activities. Subsequently, it calls super.finalize() to make sure its superclass is also finalized. The unsuspecting BaseClass calls the doLogic() method which happens to be overridden in the SubClass. This resurrects a reference to SubClass such that it is not only prevented from being garbage collected but also from using its finalizer to close new resources that may have been allocated by the called method. As detailed in MET32-J. Ensure that constructors do not call overridable methods, if the subclass's finalizer has terminated key resources, invoking its methods from the superclass might lead one to observe the object in an inconsistent state. In some cases this can result in the infamous NullPointerException.

• has executed once, the garbage collector cannot invoke it a second time.

• Garbage collection usually depends on memory availability and usage rather than on the scarcity of some other particular resource. Consequently, when memory is readily available, a scarce resource may be exhausted in spite of the presence of a finalizer that could release the scarce resource if it were executed. See FIO04-J. Release resources when they are no longer needed and TPS00-J. Use thread pools to enable graceful degradation of service during traffic bursts for more details on handling scarce resources correctly.

• It is a common myth that finalizers aid garbage collection. On the contrary, they increase garbage-collection time and introduce space overheads. Finalizers interfere with the operation of modern generational garbage collectors by extending the lifetimes of many objects. Incorrectly programmed finalizers could also attempt to finalize reachable objects, which is always counterproductive and can violate program invariants.

• Use of finalizers can introduce synchronization issues even when the remainder of the program is single-threaded. The finalize() methods are invoked by the garbage collector from one or more threads of its choice; these threads are typically distinct from the main() thread, although this property is not guaranteed. When a finalizer is necessary, any required cleanup data structures must be protected from concurrent access. See the JavaOne presentation by Hans J. Boehm [Boehm 2005] for additional information.

• Use of locks or other synchronization-based mechanisms within a finalizer can cause deadlock or starvation. This possibility arises because neither the invocation order nor the specific executing thread or threads for finalizers can be guaranteed or controlled.

Object finalizers have also been deprecated since Java 9. See MET02-J. Do not use deprecated or obsolete classes or methods for more information.

Because of these problems, finalizers must not be used in new classes.

Noncompliant Code Example (Superclass's finalizer)

Superclasses that use finalizers impose additional constraints on their extending classes. Consider an example from JDK 1.5 and earlier. The following noncompliant code example allocates a 16 MB buffer used to back a Swing JFrame object. Although the JFrame APIs lack finalize() methods, JFrame extends AWT.Frame, which does have a finalize() method. When a MyFrame object becomes unreachable, the garbage collector cannot reclaim the storage for the byte buffer because code in the inherited finalize() method might refer to it. Consequently, the byte buffer must persist at least until the inherited finalize() method for class MyFrame completes its execution and cannot be reclaimed until the following garbage-collection cycle.

class MyFrame extends JFrame {
  private byte[] buffer = new byte[16 * 1024 * 1024];
  // Persists for at least two GC cycles
}

Compliant Solution (Superclass's finalizer)

When a superclass defines a finalize() method, make sure to decouple the objects that can be immediately garbage collected from those that must depend on the finalizer. This compliant solution ensures that the buffer can be reclaimed as soon as the object becomes unreachable.

class MyFrame {
  private JFrame frame;
  private byte[] buffer = new byte[16 * 1024 * 1024]; // Now decoupled
}

Noncompliant Code Example (System.runFinalizersOnExit())

This noncompliant code example uses the System.runFinalizersOnExit() method to simulate a garbage-collection run. Note that this method is deprecated because of thread-safety issues.

According to the Java API [API 2014] class System, runFinalizersOnExit() method documentation,

Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.

The class SubClass overrides the protected finalize() method and performs cleanup activities. Subsequently, it calls super.finalize() to make sure its superclass is also finalized. The unsuspecting BaseClass calls the doLogic() method, which happens to be overridden in the SubClass. This resurrects a reference to SubClass that not only prevents it from being garbage-collected but also prevents it from calling its finalizer to close new resources that may have been allocated by the called method. As detailed in MET05-J. Ensure that constructors do not call overridable methods, if the subclass's finalizer has terminated key resources, invoking its methods from the superclass might result in the observation of an object in an inconsistent state. In some cases, this can result in NullPointerException.

class BaseClass {
  protected void finalize

class BaseClass {
  protected void finalize() throws Throwable {
    System.out.println("Superclass finalize!");
    doLogic();
  }

  public void doLogic() throws Throwable {
    System.out.println("This "Superclass finalize!");
    doLogic();
  }

  public void doLogic() throws Throwable {
    System.out.println("This is super-class!"");
  }
}

class SubClass extends BaseClass {
  private Date d; // mutableMutable instance field

  protected SubClass() {
    d = new Date();
  }

  protected void finalize() throws Throwable {
    System.out.println(""Subclass finalize!"");
    try {
      //  cleanupCleanup resources 
      d = null;				
    } finally {
      super.finalize();  // Call BaseClass's finalizer
    }
  }
	
  public void doLogic() throws Throwable {
    // anyAny resource allocations made here will persist 

    // inconsistentInconsistent object state
    System.out.println("
        "This is sub-class! The date object is: "" + d);
    // 'd' is already null
  }
}

public class BadUse {
  public static void main(String[] args) {
    try {
      BaseClass bc = new SubClass();
      // Artificially simulate finalization (do not do this)
      System.runFinalizersOnExit(true); 
    } catch (Throwable t) { 
      // Handle error 
    }  		
  }
}

This code outputs:

Subclass finalize!
Superclass finalize!
This is sub-class! The date object is: null

Compliant Solution

This compliant solution eliminates the call to the overridable doLogic() method from within the finalize() method.

class BaseClass {
  protected void finalize() throws Throwable {
    System.out.println("superclass finalize!");
    // Eliminate the call to the overridden doLogic().
  }
  ...
}

Compliant Solution (finalization)

Joshua Bloch \[[Bloch 08|AA. Java References#Bloch 08]\] suggests implementing a {{stop()}} method explicitly such that it leaves the class in an unusable state beyond its lifetime. A {{private}} field within the class can signal whether the class is unusable. All the class methods must check this field prior to operating on the class. This is akin to *EX1* discussed in [OBJ32-J. Do not allow partially initialized objects to be accessed]. As always, a good place to call the termination logic is in the {{finally}} block.

Exceptions

OBJ02-EX1: Sometimes it is necessary to use finalizers especially when working with native code. This is because the garbage collector cannot re-claim memory used by code written in another language. Also, the lifetime of the object is often unknown. Again, the native process must not perform any critical jobs that require immediate resource deallocation.

In such cases, finalize() may be used. Any subclass that overrides finalize() must explicitly invoke the method for its superclass as well. There is no automatic chaining with finalize. The correct way to handle this is shown below.

protected void finalize() throws Throwable {
  try {
    //...
  }
  finally {
    super.finalize();
  }
}

Alternatively, a more expensive solution is to declare an anonymous class so that the {{finalize()}} method is guaranteed to run for the superclass. This solution is applicable to {{public}} non-final classes. "The finalizer guardian object forces {{super.finalize}} to be called if a subclass overrides {{finalize()}} and does not explicitly call {{super.finalize}}". \[[JLS 05|AA. Java References#JLS 05]\] 

public class Foo {
  // The finalizeGuardian object finalizes the outer Foo object
  private final Object finalizerGuardian = new Object() {
    protected void finalize() throws Throwable {
    // Finalize outer Foo object
    }
  };
  //...
}

The ordering problem can be dangerous when dealing with native code. For example, if object A references object B (either directly or reflectively) and the latter gets finalized first, A's finalizer may end up dereferencing dangling native pointers. To impose an explicit ordering on finalizers, make sure that B is reachable before A's finalizer has concluded. This can be achieved by adding a reference to B in some global state variable and removing it as soon as A's finalizer gets executed. An alternative is to use the java.lang.ref references.

If a superclass defines a finalize method, make sure to decouple the objects that can be immediately garbage collected from those that depend on the finalizer. In the MyFrame example, the following code ensures that the buffer doesn't persist longer than expected.

Class MyFrame {
  private JFrame frame; 
  private byte[] buffer = new byte[16 * 1024 * 1024]; // now decoupled
}

Risk Assessment

Improper use of finalizers can result in resurrection of garbage-collection ready objects and result in denial of service vulnerabilities.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] Section 12.6, Finalization of Class Instances
\[[API 06|AA. Java References#API 06]\] [finalize()|http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#finalize()]
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 7, Avoid finalizers 
\[[Darwin 04|AA. Java References#Darwin 04]\] Section 9.5, The Finalize Method
\[[Flanagan 05|AA. Java References#Flanagan 05]\] Section 3.3, Destroying and Finalizing Objects
\[[Coomes 07|AA. Java References#Coomes 07]\] "Sneaky" Memory Retention
\[[Boehm 05|AA. Java References#Boehm 05]\] 
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 586|http://cwe.mitre.org/data/definitions/586.html] "Explicit Call to Finalize()", [CWE ID 583|http://cwe.mitre.org/data/definitions/583.html] "finalize() Method Declared Public", [CWE ID 568|http://cwe.mitre.org/data/definitions/568.html] "finalize() Method Without super.finalize()"

Joshua Bloch [Bloch 2008] suggests implementing a stop() method explicitly such that it leaves the class in an unusable state beyond its lifetime. A private field within the class can signal whether the class is unusable. All the class methods must check this field prior to operating on the class. This is akin to the "initialized flag"–compliant solution discussed in OBJ11-J. Be wary of letting constructors throw exceptions. As always, a good place to call the termination logic is in the finally block.

Exceptions

MET12-J-EX0: Finalizers may be used when working with native code because the garbage collector cannot reclaim memory used by code written in another language and because the lifetime of the object is often unknown. Again, the native process must not perform any critical jobs that require immediate resource deallocation.

Any subclass that overrides finalize() must explicitly invoke the method for its superclass as well. There is no automatic chaining with finalize. The correct way to handle it is as follows:

protected void finalize() throws Throwable {
  try {
    //...
  } finally {
    super.finalize();
  }
}

A more expensive solution is to declare an anonymous class so that the finalize() method is guaranteed to run for the superclass. This solution is applicable to public nonfinal classes. "The finalizer guardian forces super.finalize to be called if a subclass overrides finalize() and does not explicitly call super.finalize" [JLS 2015].

public class Foo {
  // The finalizeGuardian object finalizes the outer Foo object
  private final Object finalizerGuardian = new Object() {
    protected void finalize() throws Throwable {
      // Finalize outer Foo object
    }
  };
  //...
}

The ordering problem can be dangerous when dealing with native code. For example, if object A references object B (either directly or reflectively) and the latter gets finalized first, A's finalizer may end up dereferencing dangling native pointers. To impose an explicit ordering on finalizers, make sure that B remains reachable until A's finalizer has concluded. This can be achieved by adding a reference to B in some global state variable and removing it when A's finalizer executes. An alternative is to use the java.lang.ref references.

MET12-J-EX1: A class may use an empty final finalizer to prevent a finalizer attack, as specified in OBJ11-J. Be wary of letting constructors throw exceptions.

Risk Assessment

Improper use of finalizers can result in resurrection of garbage-collection-ready objects and result in denial-of-service vulnerabilities.

Automated Detection

Related Vulnerabilities

AXIS2-4163 describes a vulnerability in the finalize() method in the Axis web services framework. The finalizer incorrectly calls super.finalize() before doing its own cleanup, leading to errors in GlassFish when the garbage collector runs.

Related Guidelines

Bibliography

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Image Added Image Added Image AddedOBJ01-J. Understand how a superclass can affect a subclass      08. Object Orientation (OBJ)      OBJ03-J. Be aware that a final reference may not always refer to immutable data