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, declare either the variable must be declared as volatile or correctly synchronize the reads and writes must be synchronized correctly to the variable.
Declaring a shared variable as volatile guarantees visibility in a thread-safe manner only when both of the following conditions are met:
- A write to a variable does not depend on its current value.
- A write to a variable does not depend on the result of any non-atomic compound operations involving reads and writes of other variables. (See For more information, see CON01-J. Ensure that compound operations on shared variables are atomic for more information.)
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The first condition can be relaxed when ityou can be ensuredsure that only one thread will ever updatesupdate the value of the variable \[[Goetz 06|AA. Java References#Goetz 06]\]. However, code that relies on an invariant such as single-thread confinement to be true at all times is error-prone and difficult to maintain. This behavior is permissible under this guideline but not recommended. |
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 a somewhat higher performance overhead and can result in thread contention and deadlocks when used excessively.
Declaring a variable as volatile or correctly synchronizing the code guarantees that 64-bit primitive variables of type long and double variables are accessed atomically. (For more information on sharing long and double variables among multiple threads, see CON25-J. Ensure atomicity when reading and writing 64-bit values for information on sharing long and double variables amongst multiple threads). .)
Noncompliant Code Example (
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Non-volatile
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Flag)
This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method.
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If one thread invokes the shutdown() method to set the flag, it is possible that a second thread might not observe this that change. Consequently, the second thread may observe that done is still false and incorrectly invoke the sleep() method. A compiler is allowed to optimize the code if it determines that the value of done is never modified by the same thread, resulting in an infinite loop.
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Synchronization is a more secure alternative in situations where the volatile keyword or a java.util.concurrent.atomic.Atomic* field is inappropriate, such as if a variable's new value depends on its current value. Refer to For more information, see CON01-J. Ensure that compound operations on shared variables are atomic for more information.
Compliance with the guideline CON04-J. Synchronize using an internal private final lock object can reduce the likelihood of misuse by ensuring that untrusted callers cannot access the lock object.
Exceptions
CON00-EX1: Objects of type Class objects need not be made visible because they are created by the Virtual Machine virtual machine and their initialization always precedes any subsequent use (JMM Mailing List).
Risk Assessment
Failing to ensure the visibility of a shared primitive variable may result in a thread observing a stale value of the variable.
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The unprotected field can be observed through its graphical user interface (GUI).
Related Vulnerabilities
Search for The vulnerabilities resulting from the violation of this rule are listed on the CERT website.
References
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\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html], sectionSection 17.4.5 Happens-beforeBefore Order, sectionSection 17.4.3 Programs and Program Order, sectionSection 17.4.8 Executions and Causality Requirements \[[Bloch 08|AA. Java References#Bloch 08]\] 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 567|http://cwe.mitre.org/data/definitions/567.html] "Unsynchronized Access to Shared Data" |
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