Java Tutorials - Herong's Tutorial Examples - Version 7.03, by Dr. Herong Yang
"synchronized" - How Java Supports Synchronization
This section describes the 'synchronized' keyword and how Java supports synchronization in 3 different ways - synchronized class methods, synchronized instance methods, synchronized statement blocks.
Instead of let the programmers to design their own locks, manage the synchronization blocks, and apply the synchronization rules, Java offers a synchronization monitor on each instance of the Object class, so it can be used as a synchronization lock. Since all classes are sub classes of Object, all objects in Java can be used as synchronization locks.
Java also offers three ways to define synchronized blocks.
Synchronized Class Method:
class class_name { static synchronized type method_name() { statement block } }
All the statements in the method become the synchronized block, and the class object is the lock.
Synchronized Instance Method:
class class_name { synchronized type method_name() { statement block } }
All the statements in the method become the synchronized block, and the instance object is the lock.
Synchronized Statement:
class class_name { type method_name() { synchronized (object) { statement block } } }
All the statements specified in the parentheses of the synchronized statement become the synchronized block, and the object specified in the statement is the lock.
Java applys the synchronization rule by assigning the ownership of the lock's monitor to the threads that are running the synchronized blocks. Here is how it works:
Note that one program can have many locks and each lock can be associated with many different synchronized blocks. But the synchronization rule only applies between the synchronized block and its associated lock.
For example, the following code defines two synchronized blocks. Both are associated with the same lock, the instance object.
class class_name { type method_name() { synchronized (this) { statement block 1 } } synchronized type method_name() { statement block 2 } }
Block 1 will never be executed at the same time as block 2.
The following code defines two synchronized blocks. But they are associated with two different locks, one is the class object, and the other is the instance object. Those two synchronized blocks will never wait for each other.
class class_name { type method_name() { synchronized (this) { statement block 1 } } static synchronized type method_name() { statement block 2 } }
Last update: 2014.
Table of Contents
Execution Process, Entry Point, Input and Output
Primitive Data Types and Literals
Bits, Bytes, Bitwise and Shift Operations
Managing Bit Strings in Byte Arrays
Reference Data Types and Variables
StringBuffer - The String Buffer Class
System Properties and Runtime Object Methods
Generic Classes and Parameterized Types
Generic Methods and Type Inference
Lambda Expressions and Method References
Execution Threads and Multi-Threading Java Programs
ThreadGroup Class and "system" ThreadGroup Tree
►Synchronization Technique and Synchronized Code Blocks
Why Synchronization Is Needed in Multi-Threading Applications?
Synchronization Technique - Lock and Synchronized Code
►"synchronized" - How Java Supports Synchronization
BankingThread.java - Synchronization Sample Program
BankingThread.java - Synchronization Sample Program Output
Deadlock Condition Example Programs