Volatile: The hidden hero of Java concurrent programming

: Just work hard and leave the rest to time

: Xiaopozhan

Volatile: The invisible hero of Java concurrent programming

Preface
First: Visibility Issues
Second: Prohibit instruction reordering
Third: Happens-Before relationship
Fourth: Application cases
Fifth: Comparison of Volatile, synchronized, and Lock
- Volatile:
- synchronized:
- Lock:
- - Choose the appropriate tool:
Sixth: Common misunderstandings and precautions
- Common misunderstandings:
- Precautions:

Foreword

In today’s multi-core era, multi-thread programming has become the key to improving application performance. However, this also introduces a series of concurrency issues. This article will take you into the mysterious realm of the Volatile keyword in Java and reveal its irreplaceable role in multi-threaded programming.

First: Visibility Issues

In multi-threaded programming, visibility issues refer to situations where modifications to shared variables by one thread may not be visible to other threads. This is due to each thread having its own local memory, where they may keep copies of shared variables instead of accessing main memory directly. This may cause modifications to variables by one thread to be invisible to other threads.

The Volatile keyword can be used to solve visibility problems. When a variable is declared volatile, it tells the compiler and runtime system that the variable is shared and any modifications to it will be immediately written back to main memory, while any reads from it will be fetched from main memory. latest value.

Here is a simple example that demonstrates the visibility problem and how to solve it using Volatile:

public class VisibilityExample {<!-- -->
    private static volatile boolean flag = false;

    public static void main(String[] args) {<!-- -->
        // Thread 1: Modify the value of flag
        Thread thread1 = new Thread(() -> {<!-- -->
            try {<!-- -->
                Thread.sleep(1000); // Assume that some operations take time
            } catch (InterruptedException e) {<!-- -->
                e.printStackTrace();
            }
            flag = true; // Modify the value of flag to true
            System.out.println("Flag has been set to true.");
        });

        // Thread 2: Check the value of flag
        Thread thread2 = new Thread(() -> {<!-- -->
            while (!flag) {<!-- -->
                // Wait in a loop until flag changes to true
            }
            System.out.println("Flag is now true.");
        });

        thread1.start();
        thread2.start();
    }
}

In this example, if the flag variable is not modified with the volatile keyword, thread 2 may never exit the loop because it may never see the result of thread 1 modifying the flag. By using the volatile keyword, you can ensure that changes to the flag are visible to other threads.

Second: Disable command reordering

Instruction reordering is an optimization method adopted by modern processors to improve performance. It can change the order of execution of instructions without affecting the final execution result. However, in multithreaded programs, instruction reordering can lead to unexpected results because accesses to shared variables by different threads can be affected by the reordering.

The Volatile keyword not only guarantees visibility, but also prevents instruction reordering. In Java, when a field is declared volatile, the compiler and runtime system will ensure that read and write operations on the field will not be reordered, that is, the ordering of instructions is guaranteed.

Here is a simple example that demonstrates the problem that instruction reordering can cause and how to use Volatile to solve it:

public class ReorderExample {<!-- -->
    private static int x = 0;
    private static int y = 0;
    private static volatile boolean flag = false;

    public static void main(String[] args) {<!-- -->
        //Thread 1: write data
        Thread thread1 = new Thread(() -> {<!-- -->
            x = 1;
            y = 2;
            flag = true; //Set the flag bit to true, indicating that the data has been written
        });

        // Thread 2: Read data
        Thread thread2 = new Thread(() -> {<!-- -->
            while (!flag) {<!-- -->
                // Wait for data writing to complete
            }
            if (x == 0) {<!-- -->
                System.out.println("y: " + y); // Output the value of y
            }
        });

        thread1.start();
        thread2.start();
    }
}

In this example, if the flag variable is not modified with the volatile keyword, the compiler and processor may reorder the write operations in thread 1, causing thread 2 to start reading data before the flag becomes true. By using the volatile keyword, you can disable this reordering, ensuring that Thread 2 sees the correct results when reading the data. This helps avoid unexpected behavior in multi-threaded environments.

Third: Happens-Before relationship

The Happens-Before relationship is a rule defined in the Java Memory Model (JMM) and is used to ensure the correctness of multi-threaded programs. It defines the order of operations on shared variables, and when the results of one operation are visible to other operations. The Happens-Before relationship has the following situations:

Program Order Rule: In a thread, according to the order of program code, the previous operation happens-before the later operation.
Lock Rule: An unlock operation happens-before a subsequent lock operation on the same lock.
Volatile variable rules: A write operation to a volatile field happens-before a subsequent read operation to this field.
Thread startup rules: The start() method of the Thread object happens-before all operations of the thread.
Thread termination rules: All operations of a thread happen-before the thread terminates.
Interruption rules: A thread calls the interrupt() method of another thread Happens-Before the code of the interrupted thread detects the occurrence of an interrupt event.
Object finalization rules: The execution of an object’s constructor ends Happens-Before at the beginning of its finalize() method.

The role of the Happens-Before relationship is to provide a guarantee to ensure that programmers have a certain understanding and control of the behavior of multi-threaded programs. Especially when shared data is involved, Happens-Before relationships help avoid race conditions and other concurrency problems.

Regarding the volatile keyword, its use affects the Happens-Before relationship, which is mainly reflected in the volatile variable rules. When a thread’s write operation on a volatile variable happens-before a subsequent read operation on the variable, this means that the modification of the volatile variable by the previous thread is visible to the subsequent thread. This provides a visibility guarantee to a certain extent and prevents the impact of instruction reordering on multi-threaded programs. Therefore, using the volatile keyword helps maintain the Happens-Before relationship to better manage memory operations in multi-threaded programs.

Fourth: Application Cases

In actual scenarios, Volatile is often used in the following situations:

State identification: When a variable represents a certain state, and multiple threads need to perform corresponding operations when the state changes, volatile can be used to ensure the visibility of this state. For example, in some thread pool implementations, volatile variables can be used to identify whether the thread pool has been closed.
```
public class ThreadPool {
    private volatile boolean shutdownRequested = false;

    public void shutdown() {
        shutdownRequested = true;
    }

    public void doWork() {
        while (!shutdownRequested) {
            //Execute task
        }
    }
}
```

Double-Checked Locking: In singleton mode, in order to improve performance, double-checked locking can be used. One of the key implementation points is the use of the volatile keyword.

public class Singleton {<!-- -->
    private static volatile Singleton instance;

    private Singleton() {<!-- -->
        // private constructor
    }

    public static Singleton getInstance() {<!-- -->
        if (instance == null) {<!-- -->
            synchronized (Singleton.class) {<!-- -->
                if (instance == null) {<!-- -->
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

This ensures that the creation and initialization of Singleton instances will not cause reordering problems in a multi-threaded environment.

Best practices for using Volatile to solve concurrency issues:

Ensure visibility: Use the volatile keyword to ensure the visibility of shared variables, so that modifications to variables by one thread can be immediately seen by other threads.
Avoid compound operations that rely on volatile: Try to avoid compound operations that rely on volatile variables, because volatile can only guarantee the atomicity of a single operation, and compound operations may still have race conditions.
Clear usage scenarios: Make sure to use volatile in scenarios where you really need to ensure visibility, rather than abusing the keyword. In some cases, using locks or other concurrency tools may be more appropriate.
Understanding the Happens-Before relationship: When using volatile, understanding the Happens-Before relationship is crucial to correctly designing concurrent programs. Make sure to follow the Happens-Before rule in your program to prevent unexpected concurrency problems.
Consider alternatives: In some cases, using other concurrency tools or adopting methods such as immutable objects may be more appropriate than using volatile. Therefore, when choosing a concurrency solution, consider performance, maintainability, and security.

Fifth: Comparison of Volatile, synchronized and Lock

Below is a comparison of Volatile with two other common concurrency tools (synchronized and Lock) to help readers choose the appropriate tool to solve specific concurrency problems:

Volatile:

Advantages:

Visibility: Ensures that modifications to shared variables are visible to other threads.
Easy to use: Compared with synchronized and Lock, volatile is simpler to use and suitable for specific scenarios.

Disadvantages:

Limitedness: The atomicity of composite operations cannot be guaranteed and is applicable to single operations.
Limitations: It cannot replace the function of locking in complex concurrency scenarios.

synchronized:

Advantages:

Atomicity: Provides atomic operations on code blocks or methods.
Reentrant: Supports multiple acquisitions of the same lock by the same thread.

Disadvantages:

Relatively complex: Using synchronized requires more attention and can easily cause deadlocks and other problems.
Performance overhead: Compared with volatile, synchronized has a larger performance overhead.

Lock:

Advantages:

Flexibility: Provides more flexibility and supports more complex synchronization operations.
Interruptible: Supports interruptible lock acquisition operations to avoid deadlocks.

Disadvantages:

Complexity: Compared with synchronized, the use of Lock is more cumbersome and can easily introduce errors.
Requires manual release: When using Lock, you need to manually release the lock, which is easy to forget.

Choose the appropriate tool:

Simple visibility issues: If only simple visibility issues are involved and the atomicity of compound operations is not required, you can consider using volatile.
Basic synchronization: If you only need simple synchronization and do not need the flexibility of locks, synchronized may be a good choice.
Complex synchronization and flexibility: If you need more flexibility, such as interruptible locks, attempts to acquire locks, etc., you can choose to use Lock.
High concurrency and performance requirements: In scenarios with high concurrency and high performance requirements, the performance characteristics of various synchronization tools need to be carefully evaluated, and some concurrency frameworks or customized solutions may need to be adopted.

Overall, choosing the appropriate concurrency tool should be based on specific needs and scenarios. In some cases, a combination of tools may be needed to meet different levels of needs.

Sixth: Common misunderstandings and precautions

Programmers may make some common mistakes when using volatile. Here are some common misunderstandings and considerations:

Common misunderstandings:

Misunderstanding atomicity: volatile only ensures atomicity for a single read/write operation on a variable, but not for compound operations. For example, volatile does not ensure atomicity of increment operations.
```
// Incorrect use
private volatile int count = 0;

public void increment() {
    count + + ; // This is not an atomic operation
}
```
Composite operation issues: Since volatile cannot ensure the atomicity of composite operations, other synchronization methods need to be considered for composite operations that require atomicity, such as using synchronized< Atomic classes in /code> or java.util.concurrent.
// Incorrect use private volatile int counter = 0; public void increment() { counter + + ; // not an atomic operation }


Note:


Avoid relying on composite operations: As mentioned above, volatile does not ensure the atomicity of composite operations. If atomic compound operations are required, consider using other concurrency tools.


Understand the limitations of visibility: volatile provides visibility guarantees, but does not guarantee that all threads will see the same value at the same time. Therefore, avoid relying too much on volatile to solve all concurrency problems.


Consider concurrency strategies: volatile is mainly used to ensure visibility and does not provide mutual exclusivity. In some cases, you may need to consider other concurrency control methods, such as synchronized or Lock.


Not a replacement for all synchronization methods: volatile is not a universal replacement, and its use should be limited to scenarios where visibility is exactly required. In some cases, more complex synchronization mechanisms may be required.


Avoid over-optimization: When using volatile, do not over-optimize and make sure its visibility guarantees are truly needed and not arbitrarily added for the sake of performance.


Understand the Happens-Before rule: When using volatile, understanding the Happens-Before relationship is crucial to correctly designing concurrent programs. Make sure to follow the Happens-Before rule in your program to prevent unexpected concurrency problems.


In general, use volatile with caution and with a deep understanding of its behavior and limitations. When it comes to composite operations, mutual exclusivity, etc., other more powerful concurrency control methods need to be considered.