To answer this question, it is important to understand the concept of thread safety in the context of servlets.
Servlets in Java are designed to handle multiple requests concurrently. This means that multiple threads can access and execute the same servlet code simultaneously. Thread safety refers to ensuring that the servlet behaves correctly and consistently when accessed by multiple threads concurrently.
Now let's go through each option to understand why it is correct or incorrect:
Option A) Write servlet code to extend ThreadSafeServlet.
This option is incorrect because there is no such class called "ThreadSafeServlet" in the Java Servlet API. Extending this hypothetical class would not provide any inherent thread safety to the servlet.
Option B) Have the servlet implement SingleThreadModel.
This option is incorrect. The SingleThreadModel interface was introduced in earlier versions of the Servlet API to specify that a servlet can handle only one request at a time. However, it was deprecated in Servlet API version 2.4 and removed in version 3.0. Using SingleThreadModel is not recommended as it can cause performance issues and does not guarantee thread safety.
Option C) Synchronize the service method of the servlet.
This option is incorrect. Synchronizing the service method of the servlet would ensure that only one thread can execute the service method at a time, but it would not guarantee thread safety for the entire servlet. Other servlet methods, such as doGet(), doPost(), etc., can still be executed concurrently.
Option D) Use local variables exclusively, and if you have to use instance variables, synchronize access to them.
This option is correct. The recommended way to deal with servlets and thread safety is to use local variables exclusively. Local variables are thread-safe because each thread has its own stack frame, and local variables are stored on the stack. However, if you need to use instance variables, you should synchronize access to them. This can be done using the synchronized keyword or other thread-safe mechanisms, such as using atomic variables or locks.
The correct answer is option D. This option is correct because it promotes the use of local variables, which are inherently thread-safe, and suggests synchronizing access to instance variables to ensure thread safety.