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This noncompliant code example synchronizes on a Boolean lock object.

private final Boolean initialized = Boolean.FALSE;

public void doSomething() {
  synchronized (initialized) {
    // ...
  }
}

The Boolean type is unsuitable for locking purposes because it allows only two values: true and false. Boolean literals containing the same value share unique instances of the Boolean class in the Java Virtual Machine (JVM). In this example, initialized refers to the instance corresponding to the value false Boolean.FALSE. If any other code were to inadvertently synchronize on a Boolean literal with the this value false, the lock instance would be reused and the system could become unresponsive or could deadlock.

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This noncompliant code example locks on a boxed Integer object.

private int lockcount = 0;
private final Integer Lock = lockcount; // Boxed primitive Lock is shared

public void doSomething() {
  synchronized (Lock) {
    count++;
    // ...
  }
}

Boxed types may use the same instance for a range of integer values; consequently, they suffer from the same reuse problem as Boolean constants. The wrapper object are reused when the value can be represented as a byte; JVM implementations are also permitted to reuse wrapper objects for larger ranges of values. Note that While use of the intrinsic lock associated with the boxed Integer wrapper object is insecure; instances of the Integer object constructed using the new operator (new Integer(value)) are unique and not reused. In general, locks on any data type that contains a boxed value are insecure.

Compliant Solution (Integer)

This compliant solution recommends locking locks on a non-boxed nonboxed Integer, using a variant of the private lock object idiom. The doSomething() method synchronizes using the intrinsic lock of the Integer instance, Lock.

private int lockcount = 0;
private final Integer Lock = new Integer(lockcount);

public void doSomething() {
  synchronized (Lock) {
    count++;
    // ...
  }
}

When explicitly constructed, an Integer object has a unique reference and its own intrinsic lock that is distinct not only from other Integer objects, but also from boxed integers that have the same value. While this is an acceptable solution, it can cause maintenance problems because developers can incorrectly assume that boxed integers are also appropriate lock objects. A more appropriate solution is to synchronize on a private final lock object as described in the final compliant solution belowfor this rule.

Noncompliant Code Example (Interned String Object)

This noncompliant code example locks on an interned String object.

private final String lock = new String("LOCK").intern();

public void doSomething() {
  synchronized (lock) {
    // ...
  }
}

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According to the Java API class \[[API 2006|AA. Bibliography#API 06]\] {{java.lang.String}} documentation documentation [API 2006]:

When the intern() method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

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Additionally, hostile code from any other package can exploit this vulnerability, if the class is accessible. (For more information, see See rule LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code for more information.)

Noncompliant Code Example (String Literal)

This noncompliant code example locks on a final String literal.

// This bug was found in jetty-6.1.3 BoundedThreadPool
private final String lock = "LOCK";

// ...public void doSomething() {
  synchronized (lock) {
    // ...
  }
// ...}

String literals are constant and are automatically interned. Consequently, this example suffers from the same pitfalls as the preceding noncompliant code example.

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This compliant solution locks on a non-interned noninterned String instance.

private final String lock = new String("LOCK");

public void doSomething() {
  synchronized (lock) {
    // ...
  }
}

A String instance differs from a String literal. The instance has a unique reference and its own intrinsic lock that is distinct from other String object instances or literals. Nevertheless, a better approach is to synchronize on a private final lock object, as shown in the following compliant solution.

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This compliant solution synchronizes on a private final lock object. This is one of the few cases where in which a java.lang.Object instance is useful.

private final Object lock = new Object();

public void doSomething() {
  synchronized (lock) {
    // ...
  }
}

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A significant number of concurrency vulnerabilities arise from locking on the wrong kind of object. It is important to consider the properties of the lock object rather than indiscreetly simply scavenging for objects on which to synchronize.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] Class String, Collections
\[[Findbugs 2008|AA. Bibliography#Findbugs 08]\]
\[[Miller 2009|AA. Bibliography#Miller 09]\] Locking
\[[Pugh 2008|AA. Bibliography#Pugh 08]\] "Synchronization"
\[[Tutorials 2008|AA. Bibliography#Tutorials 08]\] [Wrapper Implementations|http://java.sun.com/docs/books/tutorial/collections/implementations/wrapper.html]

Automated Detection

The following table summarizes the examples flagged as violations by FindBugs:

Automated Detection

Some static analysis tools can detect violations of this rule.

Related Vulnerabilities

Any vulnerabilities resulting from the violation of this rule are listed on the CERT website.

Bibliography

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Image Added Image Added Image Removed      08. Locking (LCK)      Image Modified