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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. The analysis of the JDK 1.6.0 source code unveiled 31 bugs that fell into this category. \[[Pugh 08|AA. Java References#Pugh 08]\]

Noncompliant Code Example

A {{String}} constant is interned in Java. According to \[[API 06|AA. Java References#API 06]\] 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.

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 objectThus a String constant behaves like a global variable in the JVM. As demonstrated in this noncompliant example, even if each instance of an object maintains its own field lock, it points to a common String constant in the JVM. Legitimate code that locks on the same String constant will render all synchronization attempts inadequate. Likewise, hostile code from any other package can deliberately exploit this vulnerability.

//private thisfinal bugBoolean wasinitialized found in jetty-6.1.3 BoundedThreadPool
private final String _lock = "one";
synchronized(_lock) { ... }

Noncompliant Code Example

= 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 objectThis noncompliant code example synchronizes on a mutable field instead of an object and is bound to demonstrate no mutual exclusion properties, whatsoever. This is because the thread that holds a lock on the field can modify the referenced object's value which in turn will allow another thread that is blocked on the old 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 int count = 0;
private final Integer semaphoreLock = new Integer(0);
synchronized(semaphore) { ... }
 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 This is a mutual exclusion problem as opposed to the sharing issue discussed in the previous noncompliant example. Note that only 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 are unique and not reused. In general, locks on any data type that contains a boxed value can be dangerousare 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, LockIn the absence of an existing object to lock on, using a raw object to synchronize suffices.

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

public void doSomething() {
  synchronized (lockLock) {
    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 ruleNote 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.

Noncompliant Code Example

...

(Interned String Object)

This noncompliant code example locks on an interned String Synchronizing on getClass() rather than a class literal can also be counterproductive. Whenever the implementing class is subclassed, the subclass will end up locking on a completely different Class object.

synchronized(getClass()) { ... }

This idea is sometimes easy to miss, especially when one goes by the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] section 4.3.2 "The Class Object", that describes how method synchronization works:

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

Compliant Solution

Explicitly define the name of the class (superclass here) in the synchronization block. This can be achieved in two ways. Onw way is to explicitly pass the superclass's instance.Class.forName().

synchronized(SuperclassName.class) { ... }

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

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

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

Noncompliant Code Example

When using synchronization wrappers, the synchronization object needs to be the {{Collection}} object. The synchronization is necessary to enforce atomicity ([CON38-J. Ensure atomicity of thread-safe code]). This noncompliant 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 */ }
}

Risk Assessment

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 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"
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [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 AddedCON35-J. Do not try to force thread shutdown      08. Concurrency (CON)      08. Concurrency (CON)