Guidelines

Introduction

Memory that can be shared between threads is called _shared memory_ or _heap memory_. The term _variable_ as used in this section refers to both fields and array elements \[[JLS 05|AA. Bibliography#JLS 05]\]. 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, and exception handler parameters are never shared between threads and are unaffected by the [memory model|BB. Definitions#memory model].

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

The visibility of writes to shared variables can be problematic because the value of a shared variable may be cached; writing its value to main memory may be delayed. 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 difficult to discern by 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 that are inaccessible to other threads.

Initially, let a = 0 and b = 0.

VNA06-J. Do not assume that declaring a reference volatile guarantees visibility of the members of the referenced object      The CERT Oracle Secure Coding Standard for Java