18.2. Processes & Threads

On the OS level there are two most common units of execution:

• processes
• threads

Process

A process has a self-contained execution environment. In particular, it has its own memory space!

It takes longer to set up and it is usually a synonym for program/application.

Processes can communicate via Inter Process Communication (IPC) protocols supported by the OS. These are

• pipes,
• sockets.

The JVM is, in general, a single process.

Thread

We can understand a thread as a "lightweight process".

Threads exist within a process and every process has at least one thread.

Threads share the process's resources, including memory and open files. Because of that, creating a new thread requires fewer resources than creating a new process.

In Java, a program starts with a single thread, the main thread, but can create new ones.

Thread states

In Java, the life cycle of a thread is controlled by the JVM.

• New: a thread which has not yet started.
• Runnable: a thread which is executing in the Java virtual machine but may be waiting for other resources from the operating system.
• Blocked: a thread which is waiting for a monitor lock to (re)enter a synchronized block/method.
• Waiting: a thread which is waiting for another thread to perform a particular action.
• Timed Waiting: a thread which is waiting for a specified amount of time.
• Terminated: a thread which has completed execution.

Thread types

We distinguish between two thread types.

User threads

• These are high-priority threads.
• The JVM will wait for any user thread to complete its task before terminating it.
• E.g., the main thread.

Daemon threads:

• These are low-priority threads.
• The JVM will terminate all daemon threads when all user threads terminate.
• Their only role is to provide services to user threads.
• E.g. garbage collector.