LCK01-J. Do not synchronize on objects that may be reused

Misuse of synchronization primitives is a common source of concurrency issues. Synchronizing on objects that may be reused can result in deadlock and nondeterministic behavior. Consequently, programs must never synchronize on objects that may be reused.

Noncompliant Code Example (Boolean Lock Object)

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 Boolean.FALSE. If any other code were to inadvertently synchronize on a Boolean literal with this value, the lock instance would be reused and the system could become unresponsive or could deadlock.

Noncompliant Code Example (Boxed Primitive)

This noncompliant code example locks on a boxed Integer object.

private int count = 0;
private final Integer Lock = count; // 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. 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 locks on a 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 count = 0;
private final Integer Lock = new Integer(count);

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 for 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) {
    // ...
  }
}

According to the Java API class java.lang.String 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.

Consequently, an interned String object behaves like a global variable in the JVM. As demonstrated in this noncompliant code example, even when every instance of an object maintains its own lock field, the fields all refer to a common String constant. Locking on String constants has the same reuse problem as locking on Boolean constants.

Additionally, hostile code from any other package can exploit this vulnerability, if the class is accessible. 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.

Compliant Solution (String Instance)

This compliant solution locks on a 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.

Compliant Solution (Private Final Lock Object)

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

private final Object lock = new Object();

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

For more information on using an Object as a lock, see rule LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code.

Risk Assessment

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 simply scavenging for objects on which to synchronize.

Automated Detection

Some static analysis tools can detect violations of this rule.

Bibliography