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Reading a shared primitive variable in one thread may not yield the value of the most recent write to the variable from another thread. Consequently, the thread may observe a stale value of the shared variable. To ensure the visibility of the most recent update, either the variable must be declared volatile or the reads and writes must be synchronized.

Declaring a shared variable volatile guarantees visibility in a thread-safe manner only when both of the following conditions are met:

• A write to a variable is independent from its current value.
• A write to a variable is independent from the result of any nonatomic compound operations involving reads and writes of other variables (see VNA02-J. Ensure that compound operations on shared variables are atomic for more information).

The first condition can be relaxed when you can be sure that only one thread will ever update the value of the variable [Goetz 2006]. However, code that relies on a single-thread confinement is error prone and difficult to maintain. This design approach is permitted under this rule but is discouraged.

Synchronizing the code makes it easier to reason about its behavior and is frequently more secure than simply using the volatile keyword. However, synchronization has somewhat higher performance overhead and can result in thread contention and deadlocks when used excessively.

Declaring a variable volatile or correctly synchronizing the code guarantees that 64-bit primitive long and double variables are accessed atomically. For more information on sharing those variables among multiple threads, see VNA05-J. Ensure atomicity when reading and writing 64-bit values.

Noncompliant Code Example (Non-volatile Flag

Declaring shared variables as volatile ensures visibility and limits reordering of accesses. Volatile accesses do not guarantee the atomicity of composite operations such as incrementing a variable. Consequently, this recommendation is not applicable in cases where the atomicity of composite operations must be guaranteed (see CON01-J. Do not assume that composite operations on primitive data are atomic).

Declaring variables as volatile establishes a happens-before relationship such that a write to the volatile variable is always seen by a subsequent read. Statements that occur before the write to the volatile field also happen-before the read of the volatile field.

Consider two threads that are executing some statements:

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Thread 1 and Thread 2 have a happens-before relationship such that Thread 2 does not start before Thread 1 finishes. This is established by the semantics of volatile accesses.

In this example, Statement 3 writes to a volatile variable, and statement 4 (in Thread 2) reads the same volatile variable. The read sees the most recent write (to the same variable v) from statement 3.

Volatile read and write operations cannot be reordered with respect to each other and also with respect to nonvolatile variables accesses. When Thread 2 reads the volatile variable it sees the results of all the writes occurring before the write to the volatile variable in Thread 1. Because of the relatively strong guarantees of volatile, the performance overhead of volatile is almost the same as that of synchronization

However, this does not mean that statements 1 and 2 are executed in the order in which they appear in the program. They may be freely reordered by the compiler.

The possible reorderings between volatile and non-volatile variables are summarized in the matrix shown below. Load and store operations are synonymous to read and write operations, respectively. \[[Lea 08|AA. Java References#Lea 08]\]

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...

)

This noncompliant code example uses a shutdown() method to set a non-volatile the nonvolatile done flag that is checked in the run() method. If one thread invokes the shutdown() method to set the flag, it is possible that another thread might not observe this change. Consequently, the second thread may still observe that done is false and incorrectly invoke the sleep() method.:

final class ControlledStop implements Runnable {
  private boolean done = false;
 
  @Override public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        Thread.currentThread().interrupt(); // Reset handleinterrupted exceptionstatus
      } 
    } 	 
  }

  protectedpublic void shutdown() {
    done = true;
  }
}

Compliant Solution (volatile status flag)

If one thread invokes the shutdown() method to set the flag, a second thread might not observe that change. Consequently, the second thread might observe that done is still false and incorrectly invoke the sleep() method. Compilers and just-in-time compilers (JITs) are allowed to optimize the code when they determine that the value of done is never modified by the same thread, resulting in an infinite loop.

Compliant Solution (Volatile)

In this compliant solution, the done flag is declared volatile to ensure that writes This compliant solution qualifies the done flag as volatile so that updates are visible to other threads.:

final class ControlledStop implements Runnable {
  private volatile boolean done = false;
 
  @Override public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        Thread.currentThread().interrupt(); // Reset handleinterrupted exceptionstatus
      } 
    } 	 
  }

  protectedpublic void shutdown() {
    done = true;
  }
}

Noncompliant Code Example (non-volatile guard)

Compliant Solution (AtomicBoolean)

In this compliant solution, the done flag is declared to be of type java.util.concurrent.atomic.AtomicBoolean. Atomic types also guarantee that writes are visible to other threadsThis noncompliant code example declares a non-volatile variable of type int which is initialized in the constructor depending on a security check.

final class BankOperationControlledStop implements Runnable {
  private intfinal AtomicBoolean balancedone = 0;
  private boolean initialized = falsenew AtomicBoolean(false);
 
  @Override public void BankOperationrun() {
    ifwhile (!performAccountVerificationdone.get()) {
      throw new SecurityException("Invalid Account"); 
    }
    balance = 1000;   
    initialized = true; 
  }
  
  private int getBalance() try {
    if (initialized == true) {
      return balance;// ...
    }
    else {
      return -1;
    }
  }
}

The Java compiler is allowed to reorder the statements of the BankOperation() constructor so that the boolean flag initialized is set to true before the initialization has concluded. If it is possible to obtain a partially initialized instance of the class in a subclass using a finalizer attack (see OBJ04-J. Do not allow partially initialized objects to be accessed), a race condition can be exploited by invoking the getBalance() method to obtain the balance even though initialization is still underway.

Compliant Solution (volatile guard)

This compliant solution declares the initialized flag as volatile to ensure that initialization happens before the initialized flag is set.

class BankOperation {
  private int balance = 0;
  private volatile boolean initialized = falseThread.currentThread().sleep(1000); // Now volatile
 
  public BankOperation() {
    if (!performAccountVerification()) {
      throw new SecurityException("Invalid Account"); 
Do something
      }
    balance = 1000;   
    initialized = true; 
  }
  
  private int getBalance() {
    if (initialized == true) {
      return balance; catch(InterruptedException ie) { 
    }
    else {
      return -1;
    }
  }
}

Noncompliant Code Example (visibility)

This noncompliant code example consists of two classes: an immutable ImmutablePoint class and a mutable Holder class. Holder is mutable because a new ImmutablePoint instance can be assigned to it using the setPoint() method (see CON09-J. Do not assume that classes having only immutable members are thread-safe).

class Holder {
  ImmutablePoint ipoint;
  
  Holder(ImmutablePoint ip) {
   ipoint = ip;
  }
  
  ImmutablePoint getPoint() {
    return ipoint;Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public void setPointshutdown(ImmutablePoint ip) {
    this.ipoint = ip;
  }
}

public class ImmutablePoint {
  final int x;
  final int y;

  public ImmutablePoint(int x, int y) {
    this.x = x;
    this.y = ydone.set(true);
  }
}

Because the ipoint field is non-volatile, changes to this value may not be immediately visible to other threads.

Compliant Solution (

...

synchronized)

This compliant solution declares the ipoint field as volatile so uses the intrinsic lock of the Class object to ensure that updates are immediately visible to other threads. :

final class HolderControlledStop {
  volatile ImmutablePoint ipoint;
  
  Holder(ImmutablePoint ip) implements Runnable {
  private boolean ipointdone = ipfalse;
  }
  
@Override  ImmutablePoint getPoint() {
    return ipoint;
  }

  void setPoint(ImmutablePoint ippublic void run() {
    this.ipoint = ip;
  }
}

The setPoint() method does not need to be synchronized because it operates atomically on immutable data, that is, on an instance of ImmutablePoint.

Declaring immutable fields as volatile enables their safe publication in that, once published, it is impossible to change the state of the sub-object.

Noncompliant Code Example (partial initialization)

Thread-safe classes (which may not be strictly immutable), such as Container in this noncompliant code example, must declare fields as final or volatile, or synchronize all accesses of the field (see CON09-J. Do not assume that classes having only immutable members are thread-safe). In the absence of synchronization, non-final, non-volatile fields may be observed by other threads before the sub-objects' initialization has concluded.

This noncompliant code example fails to declare the map field as volatile or final. Consequently, a thread that invokes the get() method may observe the value of the map field before initialization has concluded.

public class Container<K,V> {
  Map<K,V> map;

  public synchronized void initialize() {
    if (map == null) {while (!isDone()) {
      try {
        // ...
      map = new HashMap<K,V>();	         
      Thread.currentThread().sleep(1000); // Fill some useful values into HashMapDo something
    }
  }

  public V get(Object k catch(InterruptedException ie) { 
     if (map != null) {
      return map.get(k);
    } else { Thread.currentThread().interrupt(); // Reset interrupted status
      return} null;
    }
  }
}

Compliant Solution (proper initialization)

This compliant solution declares the map field as volatile to ensure other threads see an up-to-date HashMap reference.

public class Container<K,V> {
  volatile Map<K,V> map; 	 
  }

  public synchronized voidboolean initializeisDone() {
    if (map == null) {
      map = new HashMap<K,V>();	         
      // Fill some useful values into HashMap
    }
  return done;
  }

  public Vsynchronized void getshutdown(Object k) {
    if (mapdone != null) {
      return map.get(k);
    } else {
      return null= true;
    }
  }
}

Alternative solutions to using {{volatile}} for safe publication are described in [CON26-J. Do not publish partially-constructed objects]. These alternative solutions are recommended if the values of the {{map}} can be mutated after initialization because the use of {{volatile}} only guarantees "one time safe publication" \[[Goetz 06|AA. Java References#Goetz 06]\], that is the reference is made visible only after initialization. There is no guarantee that any future updates to the map's contents will be visible immediately (see [CON11-J. Do not assume that declaring an object volatile guarantees visibility of its members] for more information). 

Risk Assessment

Although this compliant solution is acceptable, intrinsic locks cause threads to block and may introduce contention. On the other hand, volatile-qualified shared variables do not block. Excessive synchronization can also make the program prone to deadlock.

Synchronization is a more secure alternative in situations where the volatile keyword or a java.util.concurrent.atomic.Atomic* field is inappropriate, such as when a variable's new value depends on its current value (see VNA02-J. Ensure that compound operations on shared variables are atomic for more information).

Compliance with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code can reduce the likelihood of misuse by ensuring that untrusted callers cannot access the lock object.

Exceptions

VNA00-J-EX0: Class objects are created by the virtual machine; their initialization always precedes any subsequent use. Consequently, cross-thread visibility of Class objects is already assured by default.

Risk Assessment

Failing to ensure the visibility of a shared primitive variable may result in a thread observing a stale value of the variableFailing to use volatile to guarantee visibility of shared values across multiple thread and prevent reordering of accesses can result in unpredictable control flow.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation Some static analysis tools are capable of detecting violations of this rule on the CERT website.

References

\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html], section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Java Concurrency Tutorial|http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html]
\[[Lea 00|AA. Java References#Lea 00]\] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 66: Synchronize access to shared mutable data
\[[Goetz 06|AA. Java References#Goetz 06]\] 3.4.2. "Example: Using Volatile to Publish Immutable Objects"
\[[JPL 06|AA. Java References#JPL 06]\] 14.10.3. "The Happens-Before Relationship"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 667|http://cwe.mitre.org/data/definitions/667.html] "Insufficient Locking", [CWE ID 413|http://cwe.mitre.org/data/definitions/413.html] "Insufficient Resource Locking", [CWE ID 366|http://cwe.mitre.org/data/definitions/366.html]  "Race Condition within a Thread", [CWE ID 567|http://cwe.mitre.org/data/definitions/567.html]  "Unsynchronized Access to Shared Data"

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Related Guidelines

Bibliography

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Image Added Image Added Image Added11. Concurrency (CON)      11. Concurrency (CON)      CON02-J. Always synchronize on the appropriate object