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Noncompliant Code Example (Bitwise Negation)

Similarly, the The toggle() method can may also use the compound assignment operator ^= to negate the current value of flag.

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This code is also not thread-safe. A data race exists because ^= is a non-atomic nonatomic compound operation.

Noncompliant Code Example (

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Volatile)

Declaring flag volatile does not help solve the problem either:

final class Flag {
  private volatile boolean flag = true;

  public void toggle() {  // Unsafe
    flag ^= true;
  }

  public boolean getFlag() { // Safe
    return flag;
  }
}

This code remains unsuitable for multithreaded use because declaring a variable as volatile fails to does not guarantee the atomicity of compound operations on the variable.

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final class Flag {
  private boolean flag = true;

  public synchronized void toggle() {
    flag ^= true; // Same as flag = !flag;
  }

  public synchronized boolean getFlag() {
    return flag;
  }
}

This solution guards reads and writes to the flag field with a lock on the instance, that is, this. This compliant solution Furthermore, doing so ensures that changes are visible to all the threads. Now, only two execution orders are possible, one of which is shown below.in the following scenario:

The second execution order involves the same operations, but t₂ starts and finishes before t₁.
Compliance with rule 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.

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In this compliant solution, the getFlag() method is not synchronized, and flag is declared as volatile. This solution is compliant because the read of flag in the getFlag() method is an atomic operation and the volatile qualification assures visibility. The toggle() method still requires synchronization because it performs a non-atomic nonatomic operation.

final class Flag {
  private volatile boolean flag = true;

  public synchronized void toggle() {
    flag ^= true; // Same as flag = !flag;
  }

  public boolean getFlag() {
    return flag;
  }
}

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This compliant solution declares flag as an AtomicBoolean type to be of type AtomicBoolean.

import java.util.concurrent.atomic.AtomicBoolean;

final class Flag {
  private AtomicBoolean flag = new AtomicBoolean(true);

  public void toggle() {
    boolean temp;
    do {
      temp = flag.get();
    } while (!flag.compareAndSet(temp, !temp));
  }

  public AtomicBoolean getFlag() {
    return flag;
  }
}

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In this noncompliant code example, multiple threads can invoke the setValues() method to set the a and b fields. Because this class fails to test for integer overflow, users of the Adder class must ensure that the arguments to the setValues() method can be added without overflow. (For more information, see See rule NUM00-J. Detect or prevent integer overflow for more information.)

final class Adder {
  private int a;
  private int b;

  public int getSum() {
    return a + b;
  }

  public void setValues(int a, int b) {
    this.a = a;
    this.b = b;
  }
}

The getSum() method contains a race condition. For example, when a and b currently have the values 0 and Integer.MAX_VALUE, respectively, and one thread calls getSum() while another calls setValues(Integer.MAX_VALUE, 0), the getSum() method might return either 0 or Integer.MAX_VALUE, or it might overflow and wrap. Overflow will occur when the first thread reads a and b after the second thread has set the value of a to Integer.MAX_VALUE, but before it has set the value of b to 0.

Note that declaring the variables as volatile fails to resolve the issue because these compound operations involve reads and writes of multiple variables.

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The operations within the synchronized methods are now atomic with respect to other synchronized methods that lock on that object's monitor (intrinsic that is, it's intrinsic lock). It is now possible, for example, to add overflow checking to the synchronized getSum() method without introducing the possibility of a race condition.

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Automated Detection

The SureLogic Flashlight tool Some available tools can diagnose violations of this rule as by detecting instance fields with empty locksets.

The Coverity Prevent Version 5.0 ATOMICITY checker Some available static analysis tools can detect the instances of non-atomic nonatomic update of a concurrently shared value. The result of the update will be is determined by the interleaving of thread execution. The GUARDED_BY_VIOLATION checker These tools can detect the instances where thread-shared data is accessed without holding an appropriate lock, possibly causing a race condition.

Related Guidelines

Bibliography

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