Concurrency

Java supports multi-threading programming since the first version. The Java environment is inherently multi-threaded, as is the JVM. This produces additional, irreducible complexity in the behavior of Java programs. In most JVM implementations, each Java application thread corresponds precisely to a dedicated OS thread. The alternative, using a shared pool of threads to execute all Java application threads (known as green threads) proved not to provide acceptable performance profile and added needless complexity. Java current approach to multi-threading has these design principles:

• All threads in a Java process share a single, common garbage-collected heap

• Any object created by one thread can be accessed by another thread that has a reference to the object

• Objects are mutable by default: that is, values held in object fields can be changed unless the programmer explicitly uses the final keyword to mark them as immutable

Thread States

• NEW - Thread that hasn't started yet.

• RUNNABLE - Thread being executed.

• BLOCKED - Thread blocked waiting for a lock release.

• WAITING - Thread waiting using Object.wait , Thread.join or any other mechanism that makes the thread stay in a waiting state.

• TIMED_WAITING - Thread is sleeping for example with Thread.sleep.

• TERMINATED - Thread ended execution.