1.Multi-thread Two ways to create thread: extends from thread class, or implement runnable interface (prefer). Yield() and sleeping(): yield changes thread from running state to runnable state; sleeping() changes thread from running state to waiting state(); Threads can communicate with each other by wait(), notify(), notifyAll() method. Used in user interface, like when you open the interface, it takes a long time to open the whole page, in the same time you can click your mouse on some button to do something. 2.Synchronized type? synchronized methods and synchronized block. 3.what's violate? Essentially, volatile is used to indicate that a variable's value will be modified by different threads. [violate可以用来实现可见性] 4.Daemon Thread. A daemon thread is a thread, that does not prevent the JVM from exiting when the program finishes but the thread is still running. An example for a daemon thread is the garbage collection. You can use the setDaemon() method to change the Thread daemon properties.
[所谓守护线程，是指在程序运行的时候在后台提供一种通用服务的线程，比如垃圾回收线程就是一个很称职的守护者，并且这种线程并不属于程序中不可或缺的部分。因 此，当所有的非守护线程结束时，程序也就终止了，同时会杀死进程中的所有守护线程。反过来说，只要任何非守护线程还在运行，程序就不会终止。] 5.What’s difference between notify() and notifyAll()? Main difference between notify and notifyAll is that notify method will only notify one Thread and notifyAll method will notify all Threads which are waiting on that monitor or lock. By the way this is something you have been reading in all over places and to be frank, this statement despite being correct is not complete and its very difficult to understand difference between notify vs notifyAll by just reading this statement. Lot of questions comes in mind like
[具体的可以参考 notify VS notifyAll] 6.What’s the states of a thread? read state. A thread can be in one of the following states: · NEW A thread that has not yet started is in this state. · RUNNABLE A thread executing in the Java virtual machine is in this state. · BLOCKED A thread that is blocked waiting for a monitor lock is in this state. · WAITING A thread that is waiting indefinitely for another thread to perform a particular action is in this state. · TIMED_WAITING A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state. · TERMINATED A thread that has exited is in this state. 7.What is difference between forking a process and spawning a thread? Forking processes:new process will run same code as parent process but in different memory space Spawning thread: creates another independent path of execution but share same memory space
[Differs in memory space] 8.What is the relationship between threads and processes? A process can have multiple threads but a thread always belongs to a single process. Two process cannot share memory space until they are purposefully doing
inter process communication via shared memory but two threads from same process always share same memory. 9.You have thread T1, T2 and T3, how will you ensure that thread T2 run after T1 and thread T3 run after T2? Use join() method T3 calls T2. join, and T2 calls T1.join, this ways T1 will finish first and T3 will finish last. 10.What is the advantage of new Lock interface over synchronized block in Java? You need to implement a high performance cache which allows multiple reader but single writer to keep the integrity how will you implement it? The major advantage of lock interfaces on multi-threaded and concurrent programming is they provide two separate lock for reading and writing which enables you to write high performance data structure like ConcurrentHashMap and conditional blocking.
[具体的，参考 java lock interface] 11.What are differences between wait and sleep method in java? wait release the lock or monitor while sleep doesn't release any lock or monitor while waiting. Wait is used for inter-thread communication while sleep is used to introduce pause on execution. 在java.lang.Thread类中，提供了sleep()， 而java.lang.Object类中提供了wait()， sleep need try catch 12.What is difference between CyclicBarrier and CountdownLatch in Java ? One difference is that you can reuse CyclicBarrier once barrier is broken but you can not reuse ContdownLatch. Though both CyclicBarrier and CountDownLatch wait for number of threads on one or more events, main difference between them is that you can not re-use CountDownLatch once count reaches to zero, but you can reuse same CyclicBarrier even after barrier is broken. 13.Difference between Thread and Process in Java? Thread is subset of Process, in other words one process can contain multiple threads. Two process runs on different memory space, but all threads share same memory space. 14.Difference between Runnable and Callable in Java? both Runnable and Callable represent task which is intended to be executed in separate thread. Runnable is there from JDK 1.0, while Callable was added on JDK 1.5. Main difference between these two is that Callable's call() method can return value and throw Exception, which was not possible with Runnable's run() method 15.How to stop thread in Java? There was some control methods in JDK 1.0 e.g. stop(), suspend() and resume() which was deprecated in later releases due to potential deadlock threats,. To manually stop, programmers either take advantage of volatile boolean variable and check in every iteration if run method has loops or interrupt threads to abruptly cancel tasks 16.How do you share data between two thread in Java? You can share data between threads by using shared object, or concurrent data-structure like BlockingQueue. 17.Why we call start() method which in turns calls run() method, why not we directly call run() method ? Another classic java multi-threading interview question This was my original doubt when I started programming in thread. Now days mostly asked in phone interview or first round of interview at mid and junior level java interviews. Answer to this question is that, when you call start() method it creates new Thread and execute code declared in run() while directly calling run() method doesn’t create any new thread and execute code on same calling thread. 18.How will you awake a blocked thread in java? This is tricky question on threading, blocking can result on many ways, if thread is blocked on IO then I don't think there is a way to interrupt the thread, let me know if there is any, on the other hand if thread is blocked due to result of calling wait(), sleep() or join() method you can interrupt the thread and it will awake by throwing InterruptedException. 19.What is FutureTask in Java? FutureTask represents a cancellable asynchronous computation in concurrent Java application. This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation. The result can only be retrieved when the computation has completed; the get methods will block if the computation has not yet completed. A FutureTask object can be used to wrap a Callable or Runnable object. Since FutureTask also implements Runnable, it can be submitted to an Executor for execution. 20.What is thread pool? Why should you thread pool in Java? Creating thread is expensive in terms of time and resource. If you create thread at time of request processing it will slow down your response time, also there is only a limited number of threads a process can create. To avoid both of these issues, a pool of thread is created when application starts-up and threads are reused for request processing. This pool of thread is known as "thread pool" and threads are known as worker thread. From JDK 1.5 release, Java API provides Executor framework, which allows you to create different types of thread pools e.g. single thread pool, which process one task at a time, fixed thread pool (a pool of fixed number of threads) or cached thread pool (an expandable thread pool suitable for applications with many short lived tasks). See this article to learn more about thread pools in Java to prepare detailed answer of this question. 21.How do you check if a Thread holds a lock or not? There is a method called holdsLock() on java.lang.Thread, it returns true if and only if the current thread holds the monitor lock on the specified object. 22.What happens if you submit a task when the queue of the thread pool is already filled? This is another tricky question on my list. Many programmers will think that it will block until a task is cleared but its true. ThreadPoolExecutor's submit() method throws RejectedExecutionException if the task cannot be scheduled for execution. 23.What is the difference between the submit() and execute() method thread pool in Java? A main difference between the submit() and execute() method is that ExecuterService.submit()can return result of computation because it has a return type of Future, but execute() method cannot return anything because it's return type is void. Both methods are ways to submit a task to thread pools but there is a slight difference between them. execute(Runnable command) is defined in Executor interface and executes given task in future, but more importantly, it does not return anything. Its return type is void. On other hand submit() is an overloaded method, it can take either Runnable or Callable task and can return Future object which can hold the pending result of computation. This method is defined on ExecutorService interface, which extends Executor interface, and every other thread pool class e.g. ThreadPoolExecutor or ScheduledThreadPoolExecutor gets these methods. 24.Is it possible to start a thread twice? No, there is no possibility to start a thread twice. If we does, it throws an exception. 25.What about the daemon threads? The daemon threads are basically the low priority threads that provides the background support to the user threads. It provides services to the user threads. 26.What is static synchronization? If you make any static method as synchronized, the lock will be on the class not on object. 27.Thread Commutation Methods Wait() sleep Notify notifyAll() 28.Use Executor create Thread Pool public class TestThreadPool { public static void main(String[] args) { ExecutorService executor = Executors.newFixedThreadPool(5);//creating a pool of 5 threads for (int i = 0; i < 10; i++) { Runnable worker = new WorkerThread("" + i); executor.execute(worker);//calling execute method of ExecutorService } executor.shutdown(); while (!executor.isTerminated()) { } System.out.println("Finished all threads"); } } .
[转]多线程面试题
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