The solution to Java child thread cannot get Attributes (Latest recommendation)

In Java multithreading programming, developers often encounter the problem that the child thread cannot get the Attributes set by the main thread. Attributes are often used to store data related to the current thread, especially in web applications, and they are often used to manage request contexts. However, since Java threads run independently, each thread has its own memory space, one thread cannot directly access local variables or properties of another thread. This article will explore the reasons for this problem in detail and provide several effective solutions, with code examples that can be run directly.

1. Cause of the problem

In JavaThreadLocalIt is a thread local variable mechanism, allowing each thread to have its own independent variable copy, avoiding shared resource conflicts under multiple threads. In web applications, such as Spring MVC, the Attributes information for each request is usually stored inThreadLocal, which means each thread can only access its own copy of the variable. If the main thread sets some Attributes and the child thread tries to read these Attributes directly, it will not be able to get the value in the main thread becauseThreadLocalVariables and general thread properties are not shared.

2. Solution

1. Directly pass data

The most direct way is to pass the main thread's Attributes to the child thread through constructor or method parameters when creating the child thread. This method is simple and direct, and is suitable for scenarios where Attributes data is small and easy to pass.

Code Example：

import ;
import ;
class Attributes {
    private Map&lt;String, String&gt; attributes = new HashMap&lt;&gt;();
    public void setAttribute(String key, String value) {
        (key, value);
    }
    public String getAttribute(String key) {
        return (key);
    }
}
class ChildThread extends Thread {
    private Attributes attributes;
    public ChildThread(Attributes attributes) {
         = attributes;
    }
    @Override
    public void run() {
        // The child thread gets the main thread's Attributes        String value = ("key1");
        ("The value obtained by the child thread: " + value);
    }
}
public class Main {
    public static void main(String[] args) {
        Attributes attributes = new Attributes();
        ("key1", "value1");
        // Create and start child threads        ChildThread childThread = new ChildThread(attributes);
        ();
        try {
            (); // Wait for the child thread to end        } catch (InterruptedException e) {
            ();
        }
    }
}

In this example, we create aAttributesClass, used to store key-value pairs.ChildThreadClass ReceiveAttributesObject as parameter and inrunAccess data in the main thread in the method. existMainIn the class, first create aAttributesInstance and set the relevant key-value pairs, then create and start the child thread.

2. Use ThreadLocal (for thread-independent data)

If the data is thread-independent, useThreadLocalIt is a more suitable choice. AlthoughThreadLocalThe problem of child threads getting the main thread Attributes cannot be solved, but in some scenarios it provides a concise way to store thread-independent variables.

Code Example：

public class Main {
    private static ThreadLocal&lt;String&gt; threadLocal = (() -&gt; "");
    public static void main(String[] args) {
        // The main thread sets the ThreadLocal value        ("The value of the main thread");
        Thread childThread = new Thread(() -&gt; {
            // Child thread gets ThreadLocal value            String value = ();
            ("ThreadLocal value obtained by the child thread: " + value);
        });
        ();
        try {
            (); // Wait for the child thread to end        } catch (InterruptedException e) {
            ();
        }
    }
}

In this example, we useGiveThreadLocalSet an initial value. The main thread passesA value has been set. In the child thread, we use()Get the current thread'sThreadLocalvalue. It should be noted that, sinceThreadLocalThe isolation of the child thread will obtain its ownThreadLocalvalue (in this example the initial value ""), not the value set by the main thread.

3. Use InheritableThreadLocal (for parent-child thread sharing data)

In Spring MVC, if you want to share Request objects or other Attributes between parent and child threads, you can useInheritableThreadLocal。InheritableThreadLocalyesThreadLocala subclass of which allows the child thread to inherit the parent thread'sThreadLocalvariable.

However, it should be noted that onlyThreadLocalReplace withInheritableThreadLocalIt is not enough to realize data sharing between parent and child threads. It is also necessary to ensure that the parent thread'sThreadLocalThe variable has been set toinheritable=true. In Spring MVC, this usually passesMethod implementation, this method accepts aboolean inheritableparameter.

However, operate directly in the user codeRequestContextHolderandInheritableThreadLocalIt can be complex and error-prone. In practical applications, it is more common to avoid directly accessing Attributes related to HTTP requests in child threads, but to pass the required data through other methods (such as passing parameters, using shared objects, etc.).

becauseInheritableThreadLocalThe use of Spring MVC internal mechanisms involves the internal mechanism of Spring MVC, and direct operation may bring unnecessary complexity. This article does not provide specificInheritableThreadLocalCode example. However, developers can consult Spring MVC related documents or source code to understand how to use them in specific scenariosInheritableThreadLocalTo realize data sharing of parent-child threads.

3. Conclusion

In Java multithreading programming, it is a common problem for child threads to not directly access Attributes set by the main thread. This article provides two effective solutions: direct data delivery and useThreadLocal(For thread independent data). For scenarios where data needs to be shared between parent and child threads, althoughInheritableThreadLocalA possible solution is provided, but complex Spring MVC internal mechanisms may be involved in actual operation. Therefore, developers should choose the appropriate method according to specific needs and ensure the correctness and maintainability of the code.

By understanding and applying these methods, developers can better manage shared data between threads and improve program performance and stability.

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