11. Concurrency (CON)

Guidelines

CON00-J. Declare status flags as volatile

CON01-J. Do not assume that composite operations on primitive data are atomic

CON02-J. Always synchronize on the appropriate object

CON03-J. Do not use background threads during class initialization

CON04-J. Use the private lock object idiom instead of the Class object's intrinsic locking mechanism

CON05-J. Ensure that threads do not fail during activation

CON06-J. Do not defer a thread that is holding a lock

CON07-J. Do not assume that a grouping of calls to independently atomic methods is atomic

CON08-J. Do not call alien methods that synchronize on the same objects as any callers in the execution chain

CON09-J. Ensure visibility of shared references to immutable objects

CON10-J. Methods that override synchronized methods must also possess synchronization capabilities

CON11-J. Do not assume that declaring an object volatile guarantees visibility of its members

CON12-J. Avoid deadlock by requesting and releasing locks in the same order

CON13-J. Do not try to force thread shutdown

CON14-J. Do not let the "this" reference escape during object construction

CON15-J. Ensure actively held locks are released on exceptional conditions

CON16-J. Do not expect sleep() and yield() methods to have any synchronization semantics

CON17-J. Avoid using ThreadGroup APIs

CON18-J. Always invoke wait() and await() methods inside a loop

CON19-J. Use notifyAll() instead of notify() to resume waiting threads

CON20-J. Do not synchronize methods that make network calls

CON21-J. Facilitate thread reuse by using Thread Pools

CON22-J. Do not use incorrect forms of the double-checked locking idiom

CON23-J. Address the shortcomings of the Singleton design pattern

CON24-J. Use a unique channel to acquire locks on any file

CON25-J. Ensure atomicity when reading and writing 64-bit values

CON26-J. Do not publish partially-constructed objects

CON27-J. Do not execute classes that use ThreadLocal objects in a thread pool

Introduction

Memory that can be shared between threads is called shared memory or heap memory. The term variable is as used in this section refers to both fields and array elements [[JLS 05]]. Variables that are shared between threads are referred to as shared variables. All instance fields, static fields, and array elements are shared variables and are stored in heap memory. Local variables, formal method parameters, or exception handler parameters are never shared between threads and are not affected by the [memory model].

In a modern shared-memory multiprocessor architecture, each processor has one or more levels of cache that are periodically reconciled with main memory as shown in the following figure:

Because of this, the visibility of writes to shared variables can be problematic because the value of a shared variable may be cached and not written to main memory immediately. Consequently, another thread may read a stale value of the variable.

A further concern is that concurrent executions of code are typically interleaved and statements may be reordered by the compiler or runtime system to optimize performance. This results in execution orders that are not immediately obvious from an examination of the source code. Failure to account for possible reorderings is a common source of data races.

Consider the following example in which a and b are (shared) global variables or instance fields but r1 and r2 are local variables not accessible by other threads.

Initially, let a = 0 and b = 0.

Because, in Thread 1, the two assignments a = 10; and r1 = b; are not related, the compiler or runtime system is free to reorder them. Similarly in Thread 2, the statements may be freely reordered. Although it may seem counter-intuitive, the Java memory model allows a read to see a write that occurs later in the execution order.

A possible execution order showing actual assignments is:

In this ordering, r1 and r2 read the original values of the variables a and b even though they are expected to see the updated values, 10 and 20. Another possible execution order showing actual assignments is:

In this ordering, r1 and r2 read the values of a and b written from step 3 and 4, even before the statements corresponding to these steps have executed.

Restricting the set of possible reorderings makes it easier to reason about the correctness of the code.

Even if statements execute in the order the appear in a thread, caching can prevent the latest values from being reflected in the main memory.

The Java Language Specification defines the Java Memory Model (JMM) which provides certain guarantees to the Java programmer. The JMM is specified in terms of actions, which includes variable reads and writes, monitor locks and unlocks, and thread starts and joins. The JMM defines a partial ordering called happens-before on all actions within the program. To guarantee that a thread executing action B can see the results of action A, for example, there must be a happens-before relationship defined such that A happens-before B.

According to the JLS:

1. An unlock on a monitor happens-before every subsequent lock on that monitor.
2. A write to a volatile field happens-before every subsequent read of that field.
3. A call to start() on a thread happens-before any actions in the started thread.
4. All actions in a thread happen-before any other thread successfully returns from a join() on that thread.
5. The default initialization of any object happens-before any other actions (other than default-writes) of a program.
6. A thread calling interrupt on another thread happens-before
   the interrupted thread detects the interrupt
7. The end of a constructor for an object happens-before the
   start of the finalizer for that object

If a happens-before relationship does not exist between two operations, the JVM is free to reorder them. A data race occurs when a variable is written to by at least one thread and read by at least another thread, and the reads and writes are not ordered by a happens-before relationship. A correctly synchronized program is one with no data races. The Java Memory Model guarantees sequential consistency for correctly synchronized programs. Sequential consistency means that the result of any execution is the same as if the reads and writes by all threads on shared data were executed in some sequential order and the operations of each individual thread appear in this sequence in the order specified by its program [[Tanenbaum 03]]. In other words:

1. Take the read and write operations performed by each thread and put them in the order the thread executes them (thread order)
2. Interleave the operations in some way allowed by the happens-before relationships to form an execution order
3. Read operations must return most recently written data in the total program order for the execution to be sequentially consistent
4. Implies all threads see the same total ordering of reads and writes of shared variables

The actual execution order of instructions and memory accesses can be in any order as long as the actions of the thread appear to that thread as if program order were followed, and provided all values read are allowed for by the memory model. This allows the programmer to understand the semantics of the programs they write, and it allows compiler writers and virtual machine implementors to perform various optimizations [[JPL 06]].

There are several concurrency primitives that can help a programmer reason about the semantics of a multithreaded program:

volatile

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:

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]]

Risk Assessment Summary

IDS17-J. Understand how escape characters are interpreted when String literals are compiled      The CERT Sun Microsystems Secure Coding Standard for Java      VOID CON00-J. Synchronize access to shared mutable variables