The just-in-time (JIT) compiler improves the efficiency of Java code execution by compiling bytecode into machine code through the following steps: Monitoring and analysis: Identify hot methods. Compile: Compile bytecode to machine code, using machine-specific optimizations. Optimization: Further optimize the code to eliminate overhead, such as inlining methods. Advantages of JIT compilation include: Higher performance: Machine code executes faster than bytecode. Smaller memory footprint: bytecode is no longer retained after compilation. Better predictability: Machine code execution times are more predictable.

JIT Compilation in Java Virtual Machine

Introduction

Java Virtual Machine (JVM) uses a just-in-time (JIT) compiler to improve the execution efficiency of Java code. It dynamically compiles bytecode to machine code, eliminating the overhead of interpreting bytecode.

JIT compilation principle

JIT compilation is a staged process involving the following steps:

• Monitoring and analysis:JIT monitors the code that is executed when the application is running. It identifies hotspot methods, i.e. methods that are executed the most.
• Compilation:Once the hotspot method is identified, the JIT compiles the bytecode into machine code. It uses machine-specific optimization techniques to improve code efficiency.
• Optimization:JIT can further optimize code, such as eliminating overhead by inlining methods and eliminating unused code.

Advantages

JIT compilation provides the following advantages:

• Higher performance:JIT Compiled code executes much faster than interpreted bytecode.
• Smaller memory footprint:JIT compilation does not retain bytecode in memory, thus reducing memory footprint.
• Better predictability:JIT compiled code has more predictable execution times because it is machine code rather than bytecode.

Practical case

Consider the following Java method:

public int sum(int n) { int sum = 0; for (int i = 0; i < n; i++) { sum += i; } return sum; }

When the JIT compiler identifies thesummethod as a hotspot method, it compiles it into machine code. The compiled code will:

• Use registers instead of the stack to store variables.
• Use loop unrolling to reduce loop overhead.
• Inline addition operations to eliminate the overhead of method calls.

The result is a significant increase in execution speed.

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