将原子变量放入synchronized块是否有意义？（适用于Java及多语言）

Great question! Let’s break this down clearly, focusing on Java (though the core logic applies to most other languages too). We’ll tackle both parts of your inquiry one by one.

First: Does a synchronized block alone guarantee atomicity for operations inside it? #

Absolutely. The synchronized keyword enforces mutual exclusion—only one thread can execute the code inside the block at any given time. This means all operations within the block are treated as a single, indivisible unit.

Take your Example 2:

public class MyIntegerWrapper { 
    public int i; 
    public void increment() { this.i++; } 
} 
public class SharedVariables { public static MyIntegerWrapper i; } 
public class MyThread extends Thread { 
    public void run() { 
        synchronized(SharedVariables.i) { 
            i.increment(); 
        } 
    } 
}

The i++ operation is inherently non-atomic (it’s three separate steps: read current value, add 1, write back). But wrapping it in a synchronized block ensures no other thread can interrupt these steps. So the increment becomes fully atomic—you’ll never end up with lost updates from concurrent threads.

Second: Is there any practical value in putting an atomic variable inside a synchronized block? #

Most of the time, no—it’s redundant and hurts performance. But there are edge cases where it makes sense. Let’s break this down:

Redundant Case (Your Example 1) #

Your Example 1 uses an AtomicInteger’s getAndIncrement() method inside a synchronized block:

public class SharedVariables { public static AtomicInteger i; } 
public class MyThread extends Thread { 
    public void run() { 
        synchronized(SharedVariables.i) { 
            i.getAndIncrement(); 
        } 
    } 
}

The getAndIncrement() method is already atomic by design—it uses CAS (Compare-And-Swap) under the hood to perform the read-modify-write operation without locks. Adding a synchronized block here is unnecessary: it adds the overhead of a heavyweight lock (which AtomicInteger is designed to avoid) and provides no extra safety. You’re essentially paying a performance penalty for nothing.

Useful Case: Composite Atomic Operations #

The only time wrapping an atomic variable in synchronized makes sense is when you need to combine multiple atomic operations into a single, larger atomic unit.

For example, suppose you want to check the value of an AtomicInteger and update it only if it meets a condition:

synchronized(atomicCounter) {
    int current = atomicCounter.get();
    if (current < 10) {
        atomicCounter.set(current + 1);
    }
}

Without the synchronized block, two threads could both read current = 9 at the same time, both pass the condition, and both set the value to 10—resulting in a lost increment. The synchronized block ensures the entire "check-then-update" sequence is atomic, which can’t be achieved by individual atomic methods alone.

Key Takeaways #

• synchronized blocks guarantee atomicity: Any operations inside are executed mutually exclusively, so they’re atomic as a group.
• Atomic variables in synchronized blocks are usually redundant: Single atomic methods (like getAndIncrement()) don’t need extra locking—they’re already thread-safe.
• Use synchronized with atomic variables for composite logic: When you need to chain multiple atomic operations (or add conditional logic) into one atomic step, the combination is useful.

内容的提问来源于stack exchange，提问作者Luke