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Wiki MarkupCompound operations are operations that consist of more than one discrete operation. Expressions that include postfix or prefix increment ({{\+\+}}), postfix or prefix decrement ({{\-\-}}), or compound assignment operators always result in compound operations. Compound assignment expressions use operators such as {{\*=, /=, %=, \ +=, \ -=, <<=, >>=, >>>=, \ ^=}} and {{\|=}} \ [[JLS 2005|AA. References#JLS 05]\]. Compound operations on shared variables must be performed atomically to prevent [data races|BB. Glossary#data race] and [race conditions|BB. Glossary#race conditions]. and race conditions.

For information about the atomicity of a grouping of calls to independently atomic methods that belong to thread-safe classes, see rule VNA03-J. Do not assume that a group of calls to independently atomic methods is atomic.

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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 approach must not be used for getter methods that perform any additional operations other than returning the value of a volatile field without use of synchronization. Unless read performance is critical, this technique may lack significant advantages over synchronization \[ [Goetz 2006|AA. References#Goetz 06]\].

Compliant Solution (Read-Write Lock)

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final class Flag {
  private boolean flag = true;
  private final ReadWriteLock lock = new ReentrantReadWriteLock();
  private final Lock readLock = lock.readLock();
  private final Lock writeLock = lock.writeLock();

  public void toggle() {
    writeLock.lock();
    try {
      flag ^= true; // Same as flag = !flag;
    } finally {
      writeLock.unlock();
    }
  }

  public boolean getFlag() {
    readLock.lock();
    try {
      return flag;
    } finally {
      readLock.unlock();
    }
  }
}

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Read-write locks allow shared state to be accessed by multiple readers or a single writer but never both. According to Goetz \ [[Goetz 2006|AA. References#Goetz 06]\]

In practice, read-write locks can improve performance for frequently accessed read-mostly data structures on multiprocessor systems; under other conditions they perform slightly worse than exclusive locks due to their greater complexity.

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Automated Detection

Some available static analysis tools can detect the instances of nonatomic update of a concurrently shared value. The result of the update is determined by the interleaving of thread execution. These tools can detect the instances where thread-shared data is accessed without holding an appropriate lock, possibly causing a race condition.

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Bibliography

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[[API 2006]	AA. References#API 06]]	Class AtomicInteger	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="640f96f1-baa5-43a9-91df-2a70699206fc"><ac:plain-text-body><![CDATA[	[[Bloch 2008	AA. References#Bloch 08]]	Item 66. Synchronize access to shared mutable data	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="a52fac7b-c54b-4af8-bca9-65cca0e41b60"><ac:plain-text-body><![CDATA[	[[Goetz 2006	AA. References#Goetz 06]]	2.3, Locking	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="c8583ee6-f507-4866-97fc-a3e7b000d919"><ac:plain-text-body><![CDATA[	[[JLS 2005	AA. References#JLS 05]]	[Chapter 17, Threads and Locks	http://java.sun.com/docs/books/jls/third_edition/html/memory.html] ]]></ac:plain-text-body></ac:structured-macro>
	§17.4.5, Happens-Before Order
Class AtomicInteger
[Bloch 2008]	Item 66. Synchronize access to shared mutable data
[Goetz 2006]	2.3, Locking
[JLS 2005]	Chapter 17, Threads and Locks		§17.4.3, Programs and Program Order
	§17.4.8, Executions and Causality Requirements 5, Happens-Before Order
	§17.4.3, Programs and Program Order
	§17.4.8, Executions and Causality Requirements
[Lea 2000]	<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="44bf7463-35ec-43b5-a9a9-26ab6ee21312"><ac:plain-text-body><![CDATA[	[[Lea 2000	AA. References#Lea 00]]	Section 2.2.7, The Java Memory Model ]]></ac:plain-text-body></ac:structured-macro>Memory Model
	Section 2.1.1.1, Objects and Locks Objects and Locks
[Tutorials 2008]	Java Concurrency Tutorial	<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="f5dcd797-618c-422d-9694-169eaccd54d7"><ac:plain-text-body><![CDATA[	[[Tutorials 2008	AA. References#Tutorials 08]]	[Java Concurrency Tutorial	http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html]	]]></ac:plain-text-body></ac:structured-macro>

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      07. Visibility and Atomicity (VNA)