What is the difference between processes and
threads?
A process is an execution of
a program but a thread is a single
execution sequence within the process.
A process can contain multiple threads. A thread is sometimes called a
lightweight process.
Threads can be created by either
·
Extending the Thread class.
·
Implementing the Runnable interface.
The state chart diagram below describes the thread states.
Sleeping – a.k.a. Timed-waiting state
The wait( ), notify (), and notifyAll( ) methods are used to
provide an efficient way for threads to communicate with each other. These are
called only in synchronized block.
·
Calling wait
causes the current thread to wait to acquire the lock of the
Object, and
·
calling notify/notifyAll
relinquishes the lock and notify the threads waiting for that lock.
·
Thread.yeild() - Causes the currently executing thread object to
temporarily pause and allow other threads to execute.
·
t.join( ) allows
the current thread to wait indefinitely until thread “t” is finished
t.join (5000) allows
the current thread to
wait for thread “t” to finish but does not wait longer
than 5 seconds.
sleep() – Keep the
current thread into sleep hence this is in Thread class, whereas wait(
), notify (), and notifyAll( ) are
in Object class because it has to communicate between the threads.
sleep():
It
is a static method on Thread class. It makes the current thread into the
"Not
Runnable" state for specified amount of time. During this time, the thread
keeps
the lock (monitors) it has acquired.
wait():
It
is a method on Object class. It makes the current thread into the "Not
Runnable"
state.
Wait is called on a object, not a thread. Before calling wait() method, the
object
should be synchronized, means the object should be inside synchronized block.
The
call to wait() releases the acquired lock.
Q: Why thread communication
methods wait(), notify() and notifyAll() are in Object class?
In Java every Object
has a monitor and wait, notify methods are used to wait for the Object monitor
or to notify other threads that Object monitor is free now. There is no monitor
on threads in java and synchronization can be used with any Object, that’s why
it’s part of Object class so that every class in java has these essential
methods for inter thread communication.
Q: Why Thread sleep() and
yield() methods are static?
Thread sleep() and yield() methods work on the currently
executing thread. So there is no point in invoking these methods on some other
threads that are in wait state. That’s why these methods are made static so
that when this method is called statically, it works on the current executing
thread and avoid confusion to the programmers who might think that they can
invoke these methods on some non-running threads.
Q:How can we achieve thread
safety in Java?
There are several ways to achieve thread safety in java –
synchronization, atomic concurrent classes, implementing concurrent Lock
interface, using volatile keyword, using immutable classes and Thread safe
classes. Learn more at
thread
safety tutorial.
Q:What is Java Thread Dump, How
can we get Java Thread dump of a Program?
Thread dump is list of all the threads active in the JVM,
thread dumps are very helpful in analyzing bottlenecks in the application and
analyzing deadlock situations. There are many ways using which we can generate
Thread dump – Using Profiler, Kill -3 command, jstack tool etc.
Q:What is BlockingQueue? How can
we implement Producer-Consumer problem using Blocking Queue?
java.util.concurrent.BlockingQueue is a Queue that supports
operations that wait for the queue to become non-empty when retrieving and
removing an element, and wait for space to become available in the queue when
adding an element.
BlockingQueue doesn’t accept null values and throw
NullPointerException if you try to store null value in the queue.
BlockingQueue implementations are thread-safe. All queuing
methods are atomic in nature and use internal locks or other forms of
concurrency control.
ConcurrentHashMap is the
class that is similar to HashMap but works fine when you try to modify your map
at runtime.
Callable is similar to
Runnable interface but it can return any Object and able to throw Exception.
Using
Future we can find out
the status of the Callable task and get the returned Object. It provides get()
method that can wait for the Callable to finish and then return the result.
Check this post for
Callable Future Example.
Disadvantage of Synchronization:
·
can cause deadlocks when two threads are waiting on each other
to do something
·
synchronized code has the overhead of acquiring lock, which can
adversely affect the performance.
volatile variable
is one whose value is always written to and read from "main memory".
That means that different threads can access the variable.
As of Java 5, accessing a volatile variable creates a memory barrier: it
effectively synchronizes all cached copies of variables with main memory, just
as entering or exiting a synchronized block that synchronizes on a given object.
Thread Scheduler is the Operating System
service that allocates the CPU time to the available runnable threads.
Time Slicing is the process to divide the available CPU
time to the available runnable threads.
Context Switching is the process of storing
and restoring of CPU state so that Thread execution can be resumed from the
same point at a later point of time.
Thread safety is the process to make our program safe to use in
multithreaded environment, there are different ways through which we can make
our program thread safe.
- Synchronization
is the easiest and most widely used tool for thread safety in java.
- Use of Atomic
Wrapper classes from java.util.concurrent.atomic package. For example
AtomicInteger
- Use of locks
from java.util.concurrent.locks package.
- Using thread
safe collection classes, check this post for usage of ConcurrentHashMap
for thread safety.
- Using volatile
keyword with variables to make every thread read the data from memory, not
read from thread cache.
JVM
parameters in Java - JVM option it can be divided into two
parts:
1) JVM Options
that begin with -X are non-standard (thy are not guaranteed to be
supported on all JVM implementations), and are subject to change without
notice in subsequent releases of the JDK.
2) JVM Options or
parameters which are specified with -XX are not stable and are
not recommended for casual use. These options are subject to change without
notice also.
Stack size -> -Xss
Heap size -> -Xms[starting size] –Xmx[maximum size]
Q. What is
a ThreadLocal class?
A. ThreadLocal is a handy class for simplifying
development of thread-safe
concurrent programs by making the object stored in this class not sharable
between threads.
ThreadLocal class
encapsulates non-thread-safe classes to be safely used in a multi-threaded
environment and also allows you to create per-thread-singleton.
References:
