Just In Time compiler in Java

What is Just In Time compiler in Java?

The JIT or Just In Time compiler is a feature within the Java Runtime Environment (JRE) designed to enhance the performance of Java applications as they run.

Here’s how it works, broken down into simpler terms:

1. Java source code is initially compiled by the javac compiler into an intermediate form known as bytecode. This step happens before the application is run.
   
2. When the application is launched, the Java Virtual Machine (JVM) takes over and processes this bytecode.
   
3. Within the JVM, the JIT compiler plays a crucial role. It dynamically converts the bytecode into native machine code specific to the host computer’s processor. This conversion happens during the application’s execution time, rather than beforehand.
   
4. The JIT compiler doesn’t convert all the bytecode at once. Instead, it kicks into action when specific methods are called for the first time, converting those methods’ bytecode into machine code. Once a method has been compiled, the JVM bypasses the bytecode and directly executes the native machine code for subsequent calls.
   
5. This process significantly boosts the application’s performance and execution speed because running native machine code is much faster than interpreting bytecode.

In essence, the JIT compiler optimizes Java applications on the fly, ensuring they run more efficiently by translating the general-purpose bytecode into fast, system-specific instructions.

Let’s illustrate the concept of JIT compilation in the context of Java with a simple example. While we can’t directly demonstrate JIT compilation through a code snippet, as it’s a behind-the-scenes process managed by the JVM, we can simulate a scenario that benefits from JIT compilation.

Consider a Java program that performs a calculation repeatedly, such as finding the sum of the first N natural numbers. Without JIT, the JVM would interpret the bytecode for each iteration, which is slower. With JIT, after a certain threshold of executions, the JVM compiles the frequently executed code into native machine code, speeding up subsequent executions.

public class JitExample {

    // Method to calculate the sum of first N natural numbers
    public static long sumFirstNNaturalNumbers(int n) {
        long sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        return sum;
    }

    public static void main(String[] args) {
        int N = 10000; // A large N to ensure the method is called repeatedly

        // Warm-up phase to trigger JIT compilation
        for (int i = 0; i < 10000; i++) {
            sumFirstNNaturalNumbers(N);
        }

        // Timing the execution after JIT should have optimized the method
        long startTime = System.nanoTime();
        sumFirstNNaturalNumbers(N);
        long endTime = System.nanoTime();

        System.out.println("Execution time after JIT optimization (in nanoseconds): " + (endTime - startTime));
    }
}

How This Code Relates to JIT Compilation

• Warm-up Phase: The loop that calls sumFirstNNaturalNumbers(N) 10,000 times is intended to simulate a “warm-up” phase for the application. During this phase, the JVM monitors which methods are called frequently.
  
• JIT Compilation: Based on the execution profile developed during the warm-up, the JVM’s JIT compiler may decide to compile the sumFirstNNaturalNumbers method into native machine code, optimizing its execution.
  
• Measurement: After the warm-up phase, we measure the execution time of the method again. If JIT compilation has occurred, this execution should be faster than initial executions.

Caveats

• The actual performance gain and whether JIT compilation occurs for this specific method depend on several factors, including the JVM implementation, the threshold for JIT compilation, and the hardware.
  
• This example assumes that repeated execution triggers JIT compilation, but JVMs use sophisticated heuristics to decide what and when to compile. The example’s timing method is simplistic and serves to illustrate the potential effect of JIT.

This example demonstrates the principle behind JIT compilation: by compiling “hot” methods into native code, the JVM can significantly reduce the execution time of these methods in a running Java application.