Volatile or Synchronized in Java: When to Choose Which?

Volatile vs. Synchronized in Java: Understanding Memory Barriers

In Java, declaring variables as volatile and accessing them within synchronized blocks provides distinct levels of thread safety and memory visibility.

Volatile Variables:

Volatile variables enforce memory visibility, ensuring that any changes made to them are immediately reflected in main memory. This prevents other threads from seeing an outdated value of the variable. Accessing a volatile variable is non-blocking, meaning that it doesn't acquire a lock and doesn't hold up other threads.

Synchronized Blocks:

Synchronized blocks, on the other hand, provide both memory visibility and execution control. When a thread enters a synchronized block, it acquires a lock on the object associated with that block. This prevents any other thread from acquiring the same lock and executing the block concurrently. Synchronization also acts as a memory barrier, ensuring that all changes made within the block are made visible to other threads before the lock is released.

Read-Update-Write

In the context of volatile variables, "read-update-write" implies a sequence of operations where a thread reads a variable, updates its value based on that read, and writes the new value back to the variable. This sequence is not atomic under the Java Memory Model.

When to Use Volatile:

Volatile variables are suitable when:

• You need to guarantee memory visibility without the overhead of synchronization.
• You have a shared, immutable object that is recreated on the fly.
• You need to prevent reordering of critical instructions.

When to Use Synchronization:

Synchronization is more appropriate when:

• You need to control access to a shared resource to ensure thread safety.
• You are accessing a volatile variable that requires an atomic read-update-write operation.

Volatile for Variables Dependent on Input:

Using volatile for variables that depend on input can be useful if you want to ensure that other threads immediately see any changes made to the variable. However, it's important to note that volatile variables do not guarantee thread safety for operations performed on them. In your scenario, it might be better to use a synchronized block to handle input and updates atomically.

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