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The Java™ Tutorials
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Concurrency
Processes and Threads
Thread Objects
Defining and Starting a Thread
Pausing Execution with Sleep
Interrupts
Joins
The SimpleThreads Example
Synchronization
Thread Interference
Memory Consistency Errors
Synchronized Methods
Intrinsic Locks and Synchronization
Atomic Access
Liveness
Deadlock
Starvation and Livelock
Guarded Blocks
Immutable Objects
A Synchronized Class Example
A Strategy for Defining Immutable Objects
High Level Concurrency Objects
Lock Objects
Executors
Executor Interfaces
Thread Pools
Fork/Join
Concurrent Collections
Atomic Variables
Concurrent Random Numbers
For Further Reading
Questions and Exercises
Trail: Essential Classes
Lesson: Concurrency
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# Synchronization

Threads communicate primarily by sharing access to fields and the objects reference fields refer to. This form of communication is extremely efficient, but makes two kinds of errors possible: thread interference and memory consistency errors. The tool needed to prevent these errors is synchronization.

However, synchronization can introduce thread contention, which occurs when two or more threads try to access the same resource simultaneously and cause the Java runtime to execute one or more threads more slowly, or even suspend their execution. Starvation and livelock are forms of thread contention. See the section Liveness for more information.

This section covers the following topics:

- Thread Interference describes how errors are introduced when multiple threads access shared data.
- Memory Consistency Errors describes errors that result from inconsistent views of shared memory.
- Synchronized Methods describes a simple idiom that can effectively prevent thread interference and memory consistency errors.
- Implicit Locks and Synchronization describes a more general synchronization idiom, and describes how synchronization is based on implicit locks.
- Atomic Access talks about the general idea of operations that can't be interfered with by other threads.

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