Why Is Synchronizing on String Objects in Java a Bad Idea?

Synchronizing on String Objects in Java

Introduction:

In multi-threaded Java applications, synchronizing code blocks can be crucial to ensure correct and consistent behavior. However, the choice of object to synchronize upon can impact both the correctness and performance of your code. This article explores the consequences of synchronizing on String objects, a common but potentially problematic practice.

String Objects and Identity:

Unlike primitive types, String objects are immutable and acted upon by reference. While two String objects with the same value may appear identical, they are not necessarily the same object in memory.

The Issue with Synchronizing on Strings:

When multiple threads synchronize on the same String object, they are effectively serializing access to the code block protected by that synchronization. However, if the key String object is created multiple times within the program, each thread may mistakenly believe it has exclusive access to the block due to a reference mismatch.

Synchronization Example:

Consider the following example code, which attempts to synchronize access to a cache using a String key:

private SomeData[] getSomeDataByEmail(WebServiceInterface service, String email) {
    final String key = "Data-" + email;
    synchronized(key) {
        // ... //
    }
    return data;
}

In this example, even though each call to getSomeDataByEmail uses a unique String key, multiple threads may still enter the synchronization block since they are not referencing the same String object.

Solution 1: Interning String Keys

One solution is to use the intern() method on the key String object. This method ensures that only one instance of the String with a particular value exists in the JVM. By synchronizing on the interned key, you effectively guarantee that all threads are referencing the same object.

Solution 2: Using a Lock Object

A more robust solution is to create a dedicated lock object for each key. Instead of synchronizing on the key itself, threads acquire the lock associated with the key. This approach provides a consistent point of synchronization regardless of the number of key instances.

Conclusion:

Synchronizing on String objects can lead to concurrency issues if they are inadvertently created multiple times. To ensure reliable thread safety, consider using interned String keys or implementing a dedicated lock object per key. Understanding the implications of your choices for object synchronization is crucial for developing efficient and correct multi-threaded Java code.

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