Threads - Beginning Java 8 Language Features - page 218

Current thread's name: main
Current thread's group name: main
New thread's name: my new thread
New thread's group name: main

You can also create a thread group and place a new thread in that thread group. To place a new thread in your
thread group, you must use one of the constructors of the Thread class that accepts a ThreadGroup object as an
argument. The following snippet of code places a new thread in a particular thread group:

// Create a new ThreadGroup
ThreadGroup myGroup = new ThreadGroup("My Thread Group");

// Make the new thread a member of the myGroup thread group
Thread t = new Thread(myGroup, "myThreadName");

Thread groups are arranged in a tree-like structure. A thread group can contain another thread group. The
getParent() method of the ThreadGroup class returns the parent thread group of a thread group. The parent of
the top-level thread group is null .
The activeCount() method of the ThreadGroup class returns an estimate of the number of active threads in the
group. The enumerate() method of the ThreadGroup class can be used to get the threads in a thread group.
A thread group in a Java program can be used to implement a group-based policy that applies to all threads in a
thread group. For example, by calling the interrupt() method of a thread group, you can interrupt all threads in the
thread group.

Volatile Variables
I have discussed the use of the synchronized keyword in previous sections. Two things happen when a thread
executes a synchronized method/block.

• The thread must obtain the monitor lock of the object on which the method/block is

synchronized.

• The thread's working copy of the shared variables is updated with the values of those variables

in the main memory just after the thread gets the lock. The values of the shared variables in
the main memory are updated with thread's working copy value just before the thread releases
the lock. That is, at the start and at the end of a synchronized method/block, the values of the
shared variables in thread's working memory and the main memory are synchronized.

What can you do to achieve only the second point without using a synchronized method/block? That is, how
can you keep the values of variables in a thread's working memory in sync with their values in the main memory?
The answer is the keyword volatile . You can declare a variable volatile like so:

volatile boolean flag = true;

For every read request for a volatile variable, a thread reads the value from the main memory. For every write
request for a volatile variable, a thread writes the value to the main memory. In other words, a thread does not
cache the value of a volatile variable in its working memory. Note that using a volatile variable is useful only
in a multi-threaded environment for variables that are shared among threads. It is faster and cheaper than using a
synchronized block.