Java：如何生成新线程？ 不是如何生成一个新的线程来使用

Question

I don't know how Java can generate a new thread. I look into the Thread class in JDK and the init method may the key method. It seems some work done by JVM? the new Thread seems to be in the same ThreadGroup of it's parent. But I'm not sure. So I want ro know the exact flow of how Java generate a new Thread? Form JVM or other Mechanism ? and how to get the resource from the OS? Can you give me some document or site?

Answer 1

This might be JVM specific but how it works in Hotspot JVM:

1) first when you create a Thread object, which is just a regular Java object the whole state required by a native thread is set: thread-local storage, buffers, stacks, program counter etc.

2) With this in place the JVM can create a native thread. The JVM has a mapping of Thread objects and native threads so the native thread can be stopped when the Thread object gets stops.

3) As you can see underneath it's just a native OS thread so the OS takes care of scheduling.

4) When the run() method exits you can have uncaught exceptions though and therefore the JVM needs to check if it needs to be terminated or not. If no exceptions occurred all the resources from both the Thread object and the native thread are reclaimed.

Answer 2

Some more info:

Excellent JVM & Thread internals details here: http://blog.jamesdbloom.com/JVMInternals.html

A thread is a thread of execution in a program. In the Hotspot JVM there is a direct mapping between a Java Thread and a native operating system Thread. After preparing all of the state for a Java thread such as thread-local storage, allocation buffers, synchronization objects, stacks and the program counter, the native thread is created. The native thread is reclaimed once the Java thread terminates. The operating system is therefore responsible for scheduling all threads and dispatching them to any available CPU. Once the native thread has initialized it invokes the run() method in the Java thread. When the run() method returns, uncaught exceptions are handled, then the native thread confirms if the JVM needs to be terminated as a result of the thread terminating (ie is it the last non-deamon thread). When the thread terminates all resources for both the native and Java thread are released.