What happens in java if java method execution is not complete but cron scheduling makes the method run again?
In Java, if a method is still running and a new instance of the same method is triggered, either through cron scheduling or some other means, then two separate instances of the method will be running concurrently.
This can result in unexpected behavior, such as concurrent modification of shared data structures, race conditions, and other synchronization issues. It can also cause performance degradation and resource contention.
To avoid this, it's generally a good idea to ensure that a method only starts executing if its previous instance has completed. This can be done by using synchronization mechanisms such as synchronized blocks, or by using higher-level concurrency abstractions such as locks or semaphores.
In some cases, it might also be appropriate to prevent multiple instances of a method from running simultaneously by using a semaphore or a lock that ensures only one instance of the method is executing at a time.
Can you show me how to do this using synchroinzed blocks in code?
public class MyClass {
private final Object lock = new Object();
public void myMethod() {
synchronized (lock) {
// This block of code will only be executed by one thread at a time.
// The lock ensures that other threads will block until this thread has completed.
// Place your method logic here.
}
}
}In this example, a private final Object called lock is created, which is used as the lock for the synchronized block. When a thread enters the block, it acquires the lock and other threads will block until the lock is released.
This way, only one instance of myMethod will be executed at a time, ensuring that the method runs to completion before a new instance is started.
Synchronized keyword Example
The synchronized keyword in Java is used to provide a lock mechanism that ensures only one thread can execute a specific section of code (known as a critical section) at a time. When a thread enters a synchronized block, it acquires the lock for the object associated with the lock, and other threads will block until the lock is released.
The lock is associated with an object and is released automatically when the synchronized block is exited. This means that the lock is automatically released when the code execution inside the synchronized block is completed, regardless of how the block is exited (whether through normal completion, an exception, or a return statement).
/*
In this example, the incrementCount method is declared as synchronized, which means that only one thread can execute it at a time. This ensures that the count variable is protected from race conditions and other synchronization issues.
*/
public class MyClass {
private int count = 0;
public synchronized void incrementCount() {
count++;
}
public int getCount() {
return count;
}
}
synchronized can also be used with a static method to synchronize access to a shared resource that is not associated with an instance of the class. In this case, the lock is associated with the class itself instead of an object instance.
public class MyClass {
private static int count = 0;
public static synchronized void incrementCount() {
count++;
}
public static int getCount() {
return count;
}
}It's important to use synchronized carefully, as it can also cause performance issues if used excessively. Acquiring and releasing a lock involves a certain amount of overhead, and if a synchronized block is too long, it can cause a significant performance degradation. Additionally, if two synchronized blocks wait for each other to release their lock, it can cause a deadlock, where neither block can proceed.
How to make sure no deadlock happens in synchroized blocks
There is no guarantee that a deadlock will never occur in a program that uses synchronized blocks. However, there are a few best practices that can help minimize the risk of deadlocks:
1 Avoid nested synchronized blocks: If possible, avoid nesting synchronized blocks within other synchronized blocks, as this can increase the risk of deadlocks.
2 Acquire locks in a predictable order: When multiple locks are required, make sure to acquire them in a consistent and predictable order. This can help prevent deadlocks that occur when two threads each hold one lock and wait for the other to release its lock.
3 Use try-finally blocks: Use try-finally blocks to ensure that locks are always released, even if an exception is thrown.
4 Avoid holding locks for long periods of time: Avoid holding locks for long periods of time, as this increases the risk of deadlocks and can cause performance issues.
5 Use lock timeouts: Consider using lock timeouts to prevent deadlocks that occur when two or more threads wait for each other to release their lock.
Best Practice
public class MyClass {
private final Object lock1 = new Object();
private final Object lock2 = new Object();
public void method1() {
synchronized (lock1) {
try {
// Method logic goes here
} finally {
lock1.notifyAll();
}
}
}
public void method2() {
synchronized (lock2) {
try {
// Method logic goes here
} finally {
lock2.notifyAll();
}
}
}
}what does notifyAll() method do?
The notifyAll() method is a method in the java.lang.Object class and is used to signal that a particular object is no longer in use. It is typically used in conjunction with wait() method to implement synchronization between threads.
When a thread calls the wait() method on an object, it releases the lock on that object and goes to sleep until another thread calls the notify() or notifyAll() method on that same object. At that point, the sleeping thread re-acquires the lock and continues execution.
The difference between notify() and notifyAll() is that notify() wakes up one random thread that is waiting on that object, while notifyAll() wakes up all threads that are waiting on that object.
Reference
