What is thread pool? Why should we use thread pools?

Ans: A thread pool is a collection of threads on which task can be scheduled. Instead of creating a new thread for each task, you can have one of the threads from the thread pool pulled out of the pool and assigned to the task. When the thread is finished with the task, it adds itself back to the pool and waits for another assignment. One common type of thread pool is the fixed thread pool. This type of pool always has a specified number of threads running; if a thread is somehow terminated while it is still in use, it is automatically replaced with a new thread. Below are key reasons to use a Thread Pool

• Using thread pools minimizes the JVM overhead due to thread creation. Thread objects use a significant amount of memory, and in a large-scale application, allocating and de-allocating many thread objects creates a significant memory management overhead.
• You have control over the maximum number of tasks that are being processed in parallel (= number of threads in the pool).

Most of the executor implementations in java.util.concurrent use thread pools, which consist of worker threads. This kind of thread exists separately from the Runnable and Callable tasks it executes and is often used to execute multiple tasks.

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What is a thread leak? What does it mean in Java?

Ans:Thread leak is when a application does not release references to a thread object properly. Due to this some Threads do not get garbage collected and the number of unused threads grow with time. Thread leak can often cause serious issues on a Java application since over a period of time too many threads will be created but not released and may cause applications to respond slow or hang.

Q:How can I trace whether the application has a thread leak?

Ans:If an application has thread leak then with time it will have too many unused threads. Try to find out what type of threads is leaking out. This can be done using following ways:

• Give unique and descriptive names to the threads created in application. - Add log entry in all thread at various entry and exit points in threads.
• Change debugging config levels (debug, info, error etc) and analyze log messages.
• When you find the class that is leaking out threads check how new threads are instantiated and how they're closed.
• Make sure the thread is Guaranteed to close properly by doing following - Handling all Exceptions properly.
• Make sure the thread is Guaranteed to close properly by doing following

-Handling all Exceptions properly.

-releasing all resources (e.g. connections, files etc) before it closes.

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How will you take thread dump in Java? How will you analyze Thread dump?

Ans: A Thread Dump is a complete list of active threads. A java thread dump is a way of finding out what each thread in the JVM is doing at a particular point of time. This is especially useful when your java application seems to have some performance issues. Thread dump will help you to find out which thread is causing this. There are several ways to take thread dumps from a JVM. It is highly recommended to take more than 1 thread dump and analyze the results based on it. Follow below steps to take thread dump of a java process

•Step 1

On UNIX, Linux and Mac OSX Environment run below command:

ps -el | grep java

On Windows:

Press Ctrl+Shift+Esc to open the task manager and find the PID of the java process

•Step 2:

Use jstack command to print the Java stack traces for a given Java process PID

jstack [PID]

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Why are wait(), notify() and notifyAll() methods defined in the object class?

Ans: The purpose of the wait(), notify() and notifyAll() methods is to temporarily pause and resume the execution of code in an object. Typically the host object is not in a state where it can proceed with a method call it has been given and the thread of execution must literally wait for the object to return to a ready state. A common example would be a limited pool or store of objects where you must wait for a storage slot to be released or an object to be returned to the pool before you can use it.

public synchronized Object getNextObject() {

// Waiting loop

while (! objectAvailable()) {

try {

wait();

}

catch (InterruptedException e) {

// Handle exception

}

}

// No longer waiting, get the return object

Object returnObject;

// Assign the returnObject from store

// Notify state change for other waiters

notify();

return returnObject;

}

The act of waiting is associated with the Object class because any subclass may need to wait for a ready state to occur (Java is fundamentally a multi-threaded language). The waiting process acts on a single thread of execution, but the wait mechanism expects that multiple threads may be waiting for the same object. The wait() and notify() methods are hosted by the Object class so that the Java Virtual Machine can manage the “wait set” of threads through the objects they are waiting for.

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