understand multithreading in Java. • create user-defined classes with thread capability. • write multithreaded server programs. • understand the concurrent. Sun, Sun Microsystems, le logo Sun, le logo Solaris, le logo Java Coffee Cup, Common Oversights in Multithreaded Programs. The OS sits between programs and the hardware. • Contributions include: •. Uniform interaction between hardware. •. I/O abstractions (e.g., filesystems).
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JSF 2, PrimeFaces, servlets/JSP, Ajax, jQuery, Android development, Java 6 or 7 programming, custom mix of topics. – Ajax courses can concentrate on 1 library. Java is amulti threaded programming language which means we can develop multi threaded A multi threaded program contains two or more parts that can run. Two types of multithreaded applications. ▫ Make many Java threads may be created by Multi-threaded programming: more than one thread may try to use a .
Thread class. Difference between wait and sleep? Should be called from synchronized block :wait method is always called from synchronized block i.
But sleep method can be called from outside synchronized block i. IllegalMonitorStateException : if wait method is called without acquiring object lock than IllegalMonitorStateException is thrown at runtime, but sleep methodnever throws such exception.
Belongs to which class : wait method belongs to java. Object class but sleep method belongs to java. Called on object or thread : wait method is called on objects but sleep method is called on Threads not objects. And later thread scheduler schedules that thread to go from from runnable to running state. When called from synchronized block :when wait method is called thread leaves the object lock.
Differences and similarities between yield and sleep? Differences yield and sleep : Definition : yield method when called on thread gives a hint to the thread scheduler that the current thread is willing to yield its current use of a processor.
Exception : yield method need not to catch or throw any exception. But sleep method must catch or throw compile time exception i. Waiting time : yield method stops thread for unpredictable time, that depends on thread scheduler. But sleep method have got few options. Mention some g uidelines to write thread safe code, most important point we must take care of in multithreading programs? I have posted many articles regarding thread safety.
So overall this will be revision of what we have learned so far i.
Even static variables are not thread safe, they are used in static methods and if static methods are not synchronized then thread on same or different object can enter method concurrently. Multiple threads may exist on same or different objects of class but only one thread can enter static synchronized method at a time, we must consider making static methods as synchronized.
If possible, try to use volatile variables. If a field is declared volatile all threads see a consistent value for the variable. Volatile variables at times can be used as alternate to synchronized methods as well. Final variables are thread safe because once assigned some reference of object they cannot point to reference of other object.
Prentice Hall PTR - Multithreaded Programming with JAVA Technology.pdf
Using ThreadLocal :ThreadLocal is a class which provides thread-local variables. Every thread has its own ThreadLocal value that makes ThreadLocal value threadsafe as well.
Rather than StringBuffer try using immutable classes such as String. Any change to String produces new String.
How thread can enter waiting, sleeping and blocked state and how can they go to runnable state? This is very prominently asked question in interview which will test your knowledge about thread states. I will try to explain this thread state transition by framing few sub questions. I hope reading sub questions will be quite interesting.
Once specified sleep time is up thread can again return to runnable state.
Top 80 Thread- Java Interview Questions and Answers (Part 1)
In Java programming, each object has a lock; a thread can acquire the lock for an object by using the synchronized keyword. Methods, or synchronized blocks of code, can only be executed by one thread at a time for a given instantiation of a class, because that code requires obtaining the object's lock before execution.
Continuing with our copier analogy, to avoid clashing copiers, we can simply synchronize access to the copier resource, allowing only one worker access at a time, as shown in the following code sample.
We achieve this by having methods in the Copier object that modify the copier state be declared as synchronized methods. Workers that need to use a Copier object have to wait in line because only one thread per Copier object can be executing synchronized code. Why lock up an entire object, disallowing access to any other synchronized method for only brief access to shared resources?
If an object has multiple resources, it's unnecessary to lock all threads out of the whole object in order to have one thread use only a subset of the thread's resources. Semaphores Frequently, several threads will need to access a smaller number of resources. For example, imagine a number of threads running in a Web server answering client requests.
These threads need to connect to a database, but only a fixed number of available database connections are available. How can you assign a number of database connections to a larger number of threads efficiently?
One way to control access to a pool of resources rather than just with a simple one-thread lock is to use what is known as a counting semaphore. A counting semaphore encapsulates managing the pool of available resources.
Implemented on top of simple locks, a semaphore is a thread-safe counter initialized to the number of resources available for use. For example, we would initialize a semaphore to the number of database connections available.
As each thread acquires the semaphore, the number of available connections is decremented by one. Upon consumption of the resource, the semaphore is released, incrementing the counter. Threads that attempt to acquire a semaphore when all the resources managed by the semaphore are in use simply block until a resource is free. A common use of semaphores is in solving the "consumer-producer problem. The consuming thread can only obtain more data after the producing thread finishes generating it.
To use a semaphore in this manner, you create a semaphore with the initial value of zero and have the consuming thread block on the semaphore. For each unit of work completed, the producing thread signals releases the semaphore. Each time a consumer consumes a unit of data and needs another, it attempts to acquire the semaphore again, resulting in the value of the semaphore always being the number of units of completed work ready for consumption.
This approach is more efficient than having a consuming thread wake up, check for completed work, and sleep if nothing is available. Though semaphores are not directly supported in the Java language, they are easily implemented on top of object locks. Let's take a look at some common problems and their solutions: Deadlocking Deadlocking is a classic multithreading problem in which all work is incomplete because different threads are waiting for locks that will never be released.
Imagine two threads, which represent two hungry people who must share one fork and knife and take turns eating.
They each need to acquire two locks: one for the shared fork resource and one for the shared knife resource. Imagine if thread "A" acquires the knife and thread "B" acquires the fork.
Thread A will now block waiting for the fork, while thread B blocks waiting for the knife, which thread A has.
Though a contrived example, this sort of situation occurs often, albeit in scenarios much harder to detect. Although difficult to detect and hash out in every case, by following these few rules, a system's design can be free of deadlocking scenarios: Have multiple threads acquire a group of locks in the same order.
This approach eliminates problems where the owner of X is waiting for the owner of Y, who is waiting for X. Group multiple locks together under one lock. In our case, create a silverware lock that must be acquired before either the fork or knife is obtained. Label resources with variables that are readable without blocking. After the silverware lock is acquired, a thread could examine variables to see if a complete set of silverware is available. If so, it could obtain the relevant locks; if not, it could release the master silverware lock and try again later.
Most importantly, design the entire system thoroughly before writing code. Multithreading is difficult, and a thorough design before you start to code will help avoid difficult-to-detect locking problems. Volatile Variables The volatile keyword was introduced to the language as a way around optimizing compilers.
If this class were accessed by multiple threads, flag could be set by another thread after it has been set in the previous code, but before it is tested in the if statement. Declaring variables with the volatile keyword tells the compiler not to optimize out sections of code by predicting the value of the variable at compile time.
Inaccessible threads Occasionally threads have to block on conditions other than object locks. Example: In this example we are going to create a thread and explore built-in methods available for threads. Code Line 4: It overrides run method of the runnable interface as it is mandatory to override that method Code Line 6: Here we have defined the main method in which we will start the execution of the thread.
Code Line 7: Here we are creating a new thread name as "guruthread1" by instantiating a new class of thread.
Code Line 8: we will use "start" method of the thread using "guruthread1" instance.
Here the thread will start executing. Code Line Here we are using the "sleep" method of the thread using "guruthread1" instance. Hence, the thread will sleep for milliseconds. Code Here we have put sleep method in try catch block as there is checked exception which occurs i.
Interrupted exception. Code Line Here we are setting the priority of the thread to 1 from whichever priority it was Code Line Here we are getting the priority of the thread using getPriority Code Line Here we are printing the value fetched from getPriority Code Line Here we are writing a text that thread is running.
When you execute the above code, you get the following output: Output: 5 is the Thread priority, and Thread Running is the text which is the output of our code. Java Thread Synchronization In multithreading, there is the asynchronous behavior of the programs. If one thread is writing some data and another thread which is reading data at the same time, might create inconsistency in the application.
When there is a need to access the shared resources by two or more threads, then synchronization approach is utilized. Java has provided synchronized methods to implement synchronized behavior. In this approach, once the thread reaches inside the synchronized block, then no other thread can call that method on the same object. All threads have to wait till that thread finishes the synchronized block and comes out of that.
In this way, the synchronization helps in a multithreaded application. One thread has to wait till other thread finishes its execution only then the other threads are allowed for execution. Example1: GuruThread1. Code Line 8: This is the main method of the class Code Line 9: Here we are instantiating the Thread class and creating an instance named as "guruThread1" and creating a thread.
Code Line Here we are instantiating the Thread class and creating an instance named a "guruThread2" and creating a thread.How thread can enter waiting, sleeping and blocked state and how can they go to runnable state? But what about other changes that don't happen as a result of a Swing callback?
If a Swing thread is running asynchronously, responding to events and repainting the display, how can other threads modify Swing state safely? Available at public venues, or customized versions can be held on-site at your organization. Threads and the Java language To create a thread using the Java language, you instantiate an object of type Thread or a subclass and send it the start message. Although difficult to detect and hash out in every case, by following these few rules, a system's design can be free of deadlocking scenarios: Have multiple threads acquire a group of locks in the same order.
Code Line Here we are overriding start method of the runnable interface. Note: At a time one thread is executed only. For each unit of work completed, the producing thread signals releases the semaphore.
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