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Compound 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 {{\|=}} \[[JLS 052005|AA. Java References#JLS 05]\]. Compound operations on shared variables must be performed atomically to prevent [data races|BB. Definitions#data race] and [race conditions|BB. Definitions#race conditions]. |
For information about the atomicity of a grouping of calls to independently atomic methods that belong to thread-safe classes, see CON03guideline VNA03-J. Do not assume that a group of calls to independently atomic methods is atomic.
The Java Language Specification also permits reads and writes of 64-bit values to be non-atomic. For more information, see CON05guideline VNA05-J. Ensure atomicity when reading and writing 64-bit values.
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Noncompliant Code Example (volatile)
Declaring flag as volatile does not help either:
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The second execution order involves the same operations, but t2 starts and finishes before t1.
Compliance with CON07guideline 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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This approach may not be used when a getter method performs operations other than just returning the value of a {{volatile}} field without having to use any synchronization. Unless read performance is critical, this technique may not offer significant advantages over synchronization \[[Goetz 062006|AA. Java References#Goetz 06]\]. |
CON06Guideline VNA06-J. Do not assume that declaring an object reference volatile guarantees visibility of its members also addresses the volatile-read, synchronized-write pattern.
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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 \[[Goetz 062006|AA. Java References#Goetz 06]\] |
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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 does not test for integer overflow, a user of the Adder class must ensure that the arguments to the setValues() method can be added without overflow. (For more information, see guideline INT00-J. Perform explicit range checking to ensure integer operations do not overflow.)
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The getSum() method contains a race condition. For example, if 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 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.
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If operations on shared variables are not atomic, unexpected results can be produced. For example, information can be disclosed inadvertently because one user can receive information about other users.
Rule Guideline | Severity | Likelihood | Remediation Cost | Priority | Level |
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CON02 VNA02-J | medium | probable | medium | P8 | L2 |
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References
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\[[API 062006|AA. Java References#API 06]\] Class AtomicInteger \[[JLS 052005|AA. Java References#JLS 05]\] [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 \[[Tutorials 082008|AA. Java References#Tutorials 08]\] [Java Concurrency Tutorial|http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html] \[[Lea 002000|AA. Java References#Lea 00]\] Section 2.2.7 The Java Memory Model, Section 2.1.1.1 Objects and Locks \[[Bloch 082008|AA. Java References#Bloch 08]\] Item 66: Synchronize access to shared mutable data \[[Goetz 062006|AA. Java References#Goetz 06]\] 2.3. "Locking" \[[MITRE 092009|AA. Java References#MITRE 09]\] [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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