Java Processes and Threads 进程与线程

Processes and Threads

In concurrent programming, there are two basic units of execution: processes and threads. In the Java programming language, concurrent programming is mostly concerned with threads. However, processes are also important.

A computer system normally has many active processes and threads. This is true even in systems that only have a single execution core, and thus only have one thread actually executing at any given moment. Processing time for a single core is shared among processes and threads through an OS feature called time slicing.

It's becoming more and more common for computer systems to have multiple processors or processors with multiple execution cores. This greatly enhances a system's capacity for concurrent execution of processes and threads — but concurrency is possible even on simple systems, without multiple processors or execution cores.

Processes

A process has a self-contained execution environment. A process generally has a complete, private set of basic run-time resources; in particular, each process has its own memory space.

一个进程有本身独立运行的环境和资源。

Processes are often seen as synonymous with programs or applications. However, what the user sees as a single application may in fact be a set of cooperating processes. To facilitate communication between processes, most operating systems support Inter Process Communication (IPC) resources, such as pipes and sockets. IPC is used not just for communication between processes on the same system, but processes on different systems.

一个进程经常被当作为一个应用程序，可是事实上一个应用会有好多个进程，而且进程之间是能够经过IPC进行通讯的。

Most implementations of the Java virtual machine run as a single process. A Java application can create additional processes using aProcessBuilder object. Multiprocess applications are beyond the scope of this lesson.

Threads

Threads are sometimes called lightweight processes. Both processes and threads provide an execution environment, but creating a new thread requires fewer resources than creating a new process.

线程一般被看为一个轻量级的进程，进程和线程都有一个运行的环境，可是创建一个线程须要的资源更少。

Threads exist within a process — every process has at least one. Threads share the process's resources, including memory and open files. This makes for efficient, but potentially problematic, communication.

线程是依赖进程而存活的，因此一个进程至少有一个线程，线程之间共享进程的资源，包含内存等，这样更有效率可是却也潜伏着问题。

Multithreaded execution is an essential feature of the Java platform. Every application has at least one thread — or several, if you count "system" threads that do things like memory management and signal handling. But from the application programmer's point of view, you start with just one thread, called the main thread. This thread has the ability to create additional threads, as we'll demonstrate in the next section.

多线程执行是JAVA平台运行的基本特征。