When to use volatile with multi threading?

In multithreading programming, the volatile keyword is typically used to ensure that reads and writes to a variable are visible to all threads. This prevents the compiler from optimizing code involving this variable, ensuring that each access to the variable is directly from main memory rather than from the thread's local cache. The volatile keyword is particularly suitable for certain specific multithreading scenarios:

1. Status Flags

In a multithreaded environment, volatile variables are commonly used as status flags. For example, one thread monitors a condition, and other threads respond when this condition changes. A common example is stopping the execution of a thread. Suppose there is a thread running continuously, and the main thread needs to stop it at some point:

java
public class StoppableThread extends Thread {
    private volatile boolean stopRequested = false;

    public void run() {
        while (!stopRequested) {
            // Execute tasks
        }
    }

    public void requestStop() {
        stopRequested = true;
    }
}

In this example, the main thread can call the requestStop() method to update the value of the stopRequested variable. Since stopRequested is volatile, this change is visible to the StoppableThread thread, and the thread will stop safely.

2. Single Write, Multiple Reads

When a variable is written only once during its lifetime but read multiple times by multiple threads, the volatile keyword can be used. This ensures that all threads see the latest value.

java
public class Configuration {
    private volatile int configValue;

    public void setConfigValue(int value) {
        this.configValue = value;  // Single write
    }

    public int getConfigValue() {
        return configValue;  // Possible multiple reads
    }
}

In this example, once the configuration value is set via the setConfigValue method, all other threads calling getConfigValue will see the updated value.

Notes

• Not a Synchronization Mechanism: Although volatile ensures visibility of variables, it does not provide all the features of synchronization mechanisms. For example, it does not provide mutual exclusion locking or prevent instruction reordering like synchronized does.
• Limited to Variables: volatile can only be used at the variable level and does not guarantee visibility of object internal states or atomicity of compound operations. For example, increment operations (count++) are not atomic.

In summary, volatile is suitable for simple state marking of variables or scenarios with few writes and frequent reads. However, for complex synchronization or when multiple variables change together, consider using synchronized or advanced synchronization tools from the java.util.concurrent package. In Java programming, the volatile keyword is typically used with multithreading environments to ensure variable visibility and prevent instruction reordering.

Visibility

In a multithreaded program without synchronization measures, threads can cache variables in local memory. If one thread modifies the value of a variable, other threads may not see this change because they read from their own local memory copies. Using the volatile keyword ensures that when a thread modifies a variable, the new value is immediately visible to other threads. This is because the volatile keyword tells the JVM and compiler not to reorder read/write operations with other memory operations and ensures that each read/write is directly to main memory.

Example:

Suppose you have a program where one thread (the producer) continuously updates the value of a variable x, and another thread (the consumer) needs to read the latest value of x and process it. If x is not declared as volatile, the consumer thread may not see the updates made by the producer thread.

java
public class SharedObject {
    private volatile int x = 0;

    public void updateValue(int newValue) {
        x = newValue;
    }

    public int getValue() {
        return x;
    }
}

Preventing Instruction Reordering

Instruction reordering is an optimization performed by compilers and processors to improve program performance, but it can lead to unexpected behavior in multithreaded environments. The volatile keyword prevents reordering of operations involving the variable it modifies, ensuring that the execution order matches the code order.

Example:

Suppose you have two variables a and b, where b depends on the value of a. In a multithreaded environment, to ensure that operations on b see the latest value of a, declare a as volatile.

java
public class ReorderExample {
    private int a = 0;
    private volatile boolean flag = false;

    public void writer() {
        a = 1;          // 1
        flag = true;    // 2
    }

    public void reader() {
        if (flag) {     // 3
            int i = a;  // 4
            // i will be 1
        }
    }
}

In this example, flag is declared as volatile, ensuring that the operations 1 (a = 1) and 2 (flag = true) in the writer method are not reordered. This means that when flag is true, a must have been written as 1.

In summary, the volatile keyword is very useful in multithreaded programming, primarily for ensuring variable visibility and preventing instruction reordering, making multithreaded programs more secure and predictable. However, note that volatile does not provide atomicity, and for compound operations, locks or other synchronization tools should be used.