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Wiki MarkupCode that uses synchronization can sometimes be enigmatic and tricky to debug. Misuse of synchronization primitives is a common source of implementation errors. An analysis of the JDK 1.6.0 source code unveiled at least 31 bugs that fell into this category. \[[Pugh 08|AA. Java References#Pugh 08]\]

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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 non-final object that is declared public. It is possible that untrusted code can change the value of the lock object and foil any attempts to synchronizeboxed Integer object.

public Object publicLock = new Object();
synchronized(publicLock) { 
  // body
}

Compliant Solution

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, LockThis compliant solution synchronizes on a private final object and is safe from malicious manipulation.

private int count = 0;
private final ObjectInteger privateLockLock = new ObjectInteger(count);

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

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

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(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] Wiki MarkupA {{String}} constant is interned in Java. According to the Java API \[[API 06|AA. Java References#API 06]\] Class {{String}} documentation:

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, a an interned String constant object behaves like a global variable in the JVM. As demonstrated in this noncompliant code example, even if each when every instance of an object maintains its own lock field lock, it points , the fields all refer to a common String constant in the JVM. Legitimate code that locks on the same String constant renders all synchronization attempts inadequate. Likewise. Locking on String constants has the same reuse problem as locking on Boolean constants.

Additionally, hostile code from any other package can deliberately 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 = "oneLOCK";
synchronized(_lock) { /* ... */ }

Noncompliant Code Example

This noncompliant code example synchronizes on a mutable field instead of an object and demonstrates no mutual exclusion properties. This is because the thread that holds a lock on the field can modify the referenced object's value which allows another thread that is blocked on the unmodified value to resume, at the same time, granting access to a third thread that is blocked on the modified value. When aiming to modify a field, it is incorrect to synchronize on the same (or another) field as this is equivalent to synchronizing on the field's contents.

private Integer semaphore = new Integer(0);
synchronized(semaphore) { /* ... */ }

This is a mutual exclusion problem as opposed to the sharing issue discussed in the previous noncompliant code example. Note that the boxed Integer primitive is shared as shown below and not the Integer object (new Integer(value)) itself.

int lock = 0;
Integer Lock = lock; // Boxed primitive Lock will be shared

In general, holding a lock on any data structure that contains a boxed value can be dangerous.

Noncompliant Code Example

This noncompliant code example uses a {{Boolean}} field to synchronize. However, there can only be two possible values ({{true}} and {{false}}) that a {{Boolean}} can assume. Consequently, any other code that synchronizes on the same value can cause unresponsiveness and deadlocks \[[Findbugs 08|AA. Java References#Findbugs 08]\].

private Boolean initialized = Boolean.FALSE;
synchronized(initialized) { 
  if (!initialized) {
    // Perform initialization
    initialized = Boolean.TRUE;
  }
}

Compliant Solution

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 instanceIn the absence of an existing object to lock on, using a raw object to synchronize suffices.

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

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

Note that the instance of the raw object should not be changed from within the synchronized block. For example, creating and storing the reference of a new object into the lock field is highly inadvisable. To prevent such modifications, declare the lock field final.

Noncompliant Code Example

Synchronizing on getClass() rather than a class literal can also be counterproductive. Whenever the implementing class is subclassed, the subclass locks on a completely different Class object.

synchronized(getClass()) { /* ... */ }

This idea is sometimes easy to miss, especially when the Java Language Specification is misunderstood. Section 4.3.2 "The Class Object" of the specification \[[JLS 05|AA. Java References#JLS 05]\] describes how method synchronization works:

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

This does not mean that it is required to synchronize on the Class object of the base class.

Compliant Solution

Explicitly define the name of the class (superclass in this example) in the synchronization block. This can be achieved in two ways. One way is to explicitly pass the superclass's instance.

synchronized(SuperclassName.class) { ... }

}

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 usefulThe second way is to use the Class.forName() method.

synchronized(Class.forName("SuperclassName")) { ... }

Finally, it is more important to recognize the entities with whom synchronization is required rather than indiscreetly scavenging for variables or objects to synchronize on.

Noncompliant Code Example

When using synchronization wrappers, the synchronization object must be the {{Collection}} object. The synchronization is necessary to enforce atomicity ([CON06-J. Ensure atomicity of thread-safe code]). This noncompliant code example demonstrates inappropriate synchronization resulting from locking on a {{Collection}} view instead of the Collection itself \[[Tutorials 08|AA. Java References#Tutorials 08]\]. 

Map<Integer, String> m = Collections.synchronizedMap(new HashMap<Integer, String>());
Set<Integer> s = m.keySet();
synchronized(s) {  // Incorrectly synchronizes on s
  for(Integer k : s) { 
    // Do something 
  }
}

Compliant Solution

This compliant solution correctly synchronizes on the Collection object instead of the Collection view.

// ...
synchronized(m) {  // Synchronize on m, not s
  for(Integer k : s) { 
    // Do something  
  }
}

Noncompliant Code Example

This noncompliant code example incorrectly uses a ReentrantLock as the lock object.

final Lock lock = new ReentrantLock();
synchronized(lock) { /* ... */ }

Compliant Solution

The proper mechanism to lock in this case is to explicitly use the lock() and unlock() methods provided by the ReentrantLock class.

final Lock lock = new ReentrantLock();
lock.lock();
try {
  // ...
} finally {
  lock.unlock();
}

Risk Assessment

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 synchronizeSynchronizing on an incorrect variable can provide a false sense of thread safety and result in nondeterministic behavior.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] Class String
\[[Pugh 08|AA. Java References#Pugh 08]\] "Synchronization"
\[[Miller 09|AA. Java References#Miller 09]\] Locking
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Wrapper Implementations|http://java.sun.com/docs/books/tutorial/collections/implementations/wrapper.html]

Some static analysis tools can detect violations of this rule.

Bibliography

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Image Added Image Added Image AddedVOID CON00-J. Synchronize access to shared mutable variables      11. Concurrency (CON)      CON02-J. Do not use background threads during class initialization