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Monday, August 30, 2010


8:40:00 PM Posted by Satish Kumar , , 1 comment

When it comes to java thread, I always try to dig how the Thread.run() method get executed when, the Thread.start() method get called. Recently, I downloaded the JDK 5 source code and started looking into the implementation.

There is two way we can create user defined threads and in both the scenarios, we used to call the Thread constructor and that internally calls the Thread.init(). init() has the following signature. Basically this method initialize the Thread class members like group, daemon, priority, name etc.

private void init(ThreadGroup paramThreadGroup, Runnable paramRunnable, String paramString, long paramLong);

Now the actual game begins, when you call the start() on a thread object. start() method adds the thread to a thread group as per the initializing parameter. And here the start() calls a native method start0() after that. To find out further, I started looking at the Thread's native implementation.Starting with \JDK 1.5 Source\j2se\src\share\native\java\lang\Thread.c, I found the start0() method is mapped to (void *)&JVM_StartThread (refer line no 25). In \JDK 1.5 Source\j2se\src\share\javavm\export\jvm.h from line no 206 to 256 JNI configuration is there for Thread class native implementation.

Now it is time to jump to the implementation of (void *)&JVM_StartThread method. \JDK 1.5 Source\hotspot\src\share\vm\prims\jvm.cpp contains the implementation of above method. This method basically create a native Java thread and starts the same (refer from line no. 2257 to 2314).

To see the native thread start method, I went to \JDK 1.5 Source\hotspot\src\share\vm\runtime\thread.cpp. In the Thread::start(Thread* thread) method, the particular thread is initialized to RUNNABLE state followed by passing the thread reference to os.

In \JDK 1.5 Source\hotspot\src\share\vm\runtime\os.cpp, the os::start_thread(Thread* thread) method creates a OS thread.

As I have limited exposure in C/C++, I could not move further. But I discussed this in some of the Java forums and found that there will be different thread implementation for different OS and once the OS thread is created, the OS calls and manages the Thread.run() method according to it's own mechanism.

To move further in this, I was needing a C/C++ developer and could probably have looked in to the OS level. I started looking in to Ubuntu, but got confused. So finally left that there.  

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