Thread Model in Java is built around shared memory and Locking!
Concurrency basically means two or more tasks happen in parallel and they compete to access a resource. In the object-oriented world, the resource would be an object which could abstract a database, file, socket, network connection etc. In the concurrent environment, multiple threads try to get hold of the same resource. Locks are used to ensuring consistency in case there is a possibility of concurrent execution of a piece of code. Let's cover these aspects briefly:
Concurrency basically means two or more tasks happen in parallel and they compete to access a resource. In the object-oriented world, the resource would be an object which could abstract a database, file, socket, network connection etc. In the concurrent environment, multiple threads try to get hold of the same resource. Locks are used to ensuring consistency in case there is a possibility of concurrent execution of a piece of code. Let's cover these aspects briefly:
Concurrent Execution is about
- Mutual Exclusion or Mutex
- Memory Consistency or Visibility of change
Mutual Exclusion
Mutual exclusion ensures that at a given point of time only one thread can modify a resource. If you write an algorithm which guarantees that a given resource can be modified by (only) one thread then mutual exclusion is not required.
Visibility of Change
This is about propagating changes to all threads. If a thread modifies the value of a resource and then (right after that) another thread wants to read the same then the thread model should ensure that read thread/task gets the updated value.
The most costly operation in a concurrent environment contends write access. Write access to a resource, by multiple threads, requires expensive and complex coordination. Both read as well as write requires that all changes are made visible to other threads.
Locks
Locks provide mutual exclusion and ensure that visibility of change is guaranteed (Java implements locks using the synchronized keyword which can be applied on a code block or method).
Read about the cost of locks, here.
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