Object not locked by thread before wait


Threads

A thread is a unit of execution within a process. A process can have multiple threads, all of which execute code simultaneously. When multiple threads try to access the same resource, they need to be synchronized so that the resource is not corrupted or destroyed.

What are threads?

In computer science, a thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system. The implementation of threads and processes differs between operating systems, but in most cases, a thread is a component of a process. Multiple threads can exist within one process, executing concurrently and sharing resources such as memory, while different processes do not share these resources. In particular, the threads of a process share its executable code and the values of its variables at any given time.

What is the difference between a process and a thread?

A process is an instance of a program that is being executed by the CPU. A thread is a unit of execution within a process.

A process can have multiple threads, which can be executing concurrently. In fact, most modern operating systems allow processes to have multiple threads. Threads within the same process share certain resources, such as memory space.

Locking

Locking is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. A lock can be exclusive or shared. An exclusive lock, also known as a mutex, allows only one thread to access the resource. A shared lock, also called a monitor, allows multiple threads to have access to the resource.

What is locking?

Locking is a mechanism for controlling access to resources in a multithreaded environment. Locks allow threads to cooperate by preventing other threads from accessing locked resources. All locks implement the java.util.concurrent.locks.Lock interface.

The Lock interface provides two methods:
lock() and unlock().
Threads call lock() to acquire a lock and unlock() to release it. Once a thread has acquired a lock, it can access the locked resource freely until it calls unlock(). Other threads that try to call lock() will block until the thread that has the lock calls unlock(), at which point one of them will be able to proceed.

Locks also support a tryLock() method that tries to acquire a lock but does not block if the lock is not available. The return value of tryLock() indicates whether the lock was acquired or not.

What are the benefits of locking?

There are several benefits to locking:
-Locking can prevent two threads from conflicting with each other by ensuring that only one thread can access a resource at a time.
-Locking can improve performance by allowing the thread that holds the lock to execute code without having to constantly check if another thread has acquired the lock.
-Locking can provide visibility guarantees so that one thread cannot access a resource that another thread is in the process of modifying.

What are the drawbacks of locking?

As with any synchronization mechanism, there are always going to be some drawbacks to using locks. In particular, locks can lead to two main problems:

  1. Deadlock
  2. Livelock
    Wait
    Every object has a lock. A lock is like a switch that can be turned on or off. When the lock is “on,” only one thread can execute code in the object. When the lock is “off,” any number of threads can execute code in the object.
    What is wait?

Wait is a command line utility that allows you to schedule commands or scripts to run at a later time. It is useful for executing recurring tasks, such as backing up data or checking for system updates. Wait is included in most Unix-like operating systems, such as Linux and macOS.

What are the benefits of wait?

There are a number of benefits associated with wait(). First, it allows the thread to release the lock on the object, which means that other threads can access the object. Second, it allows the thread to go to sleep until it is woken up by another thread (via notify() or notifyAll()). This can be useful if the thread is waiting for something to happen (such as a file to be read or a database query to complete) and there is no need for it to be actively doing anything. Finally, wait() is interruptible, which means that a thread can be woken up early if needed (for example, if the program needs to shut down).

What are the drawbacks of wait?

There are a few drawbacks associated with the wait method. First, it is possible for the waiting thread to remain in a WAITING state indefinitely if the notify or notifyAll method is not called by another thread. This can happen if the object is locked by a different thread, or if the notify method is called but there are multiple threads waiting, and the notify method only wakes up one waiting thread. Another drawback is that, like sleep, wait is a static method, so it must be called from within a synchronized block or method; if it is not, an IllegalMonitorStateException will be thrown. Finally, since wait releases the lock on an object, other threads may modify the object’s state between the time that the current thread releases the lock and the time that it re-acquires the lock. This could lead to data inconsistency errors.

Use Cases

The Java Thread class has a number of methods for dealing with concurrency, including wait(), notify(), and notifyAll(). The wait() method is used to cause a thread to wait until it is notified or interrupted. The notify() and notifyAll() methods are used to notify waiting threads that an event has occurred.

When should you use locking?

There are many reasons why you might want to lock a section of code. The most common reason is to prevent two threads from accessing the same data at the same time. This can lead to data corruption if both threads try to modify the data simultaneously. Locking can also be used to ensure that a thread does not begin executing a section of code until another thread has finished with it.

Other reasons for using locking include:

  • Ensuring that a thread does not begin executing a section of code until another thread has finished with it
  • Preventing two threads from accessing the same data at the same time
  • Guaranteeing the order in which two threads access shared resources
  • Allowing a thread to wait for another thread to perform an action
  • Improving performance by allowing concurrent access to shared resources
    When should you use wait?
    You should only use wait if the object is locked by the current thread. Otherwise, you will get an IllegalMonitorStateException.
    Conclusion
    Deadlocks can occur in any situation where two or more threads are blocked waiting for each other to do something. In general, a deadlock occurs when each thread is waiting for a resource that the other thread is holding.

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