Just In Time Compiler - Java

Why Is JIT Important in Java?

The Just-In-Time (JIT) compiler is a critical component of the Java Virtual Machine (JVM) that significantly enhances Java application performance. Unlike traditional interpreters, the JIT compiler converts bytecode into native machine code at runtime, allowing Java programs to run faster and more efficiently. Below are three key reasons that highlight the importance of JIT in Java.

1. Improves Runtime Performance
The JIT compiler boosts Java performance by translating bytecode into native machine code during execution. This native code runs much faster than interpreted bytecode. The JIT identifies hot code paths—sections of code that run frequently—and compiles them once for reuse, reducing processing overhead. As a result, the more a method is used, the faster it executes. This Java performance optimization makes JIT ideal for long-running or resource-intensive applications.

2. Enables Adaptive Optimization
One of the major JIT compiler benefits is its ability to optimize code dynamically based on real-time behavior. The JIT tracks which methods or loops are used most often and applies smart optimizations like method inlining, loop unrolling, and branch prediction. This adaptive optimization ensures the JVM makes intelligent decisions that improve speed and reduce redundant calculations. It’s like an “on-the-fly translator” that keeps refining code as the program runs.

3. Reduces Recompilation and Startup Delays Compared to AOT
Unlike Ahead-of-Time (AOT) compilation, which compiles the entire codebase before execution, JIT compiles only what’s needed at runtime. This approach allows applications to start faster and get optimized gradually during use. By avoiding full upfront compilation, JIT reduces initial load time while still benefiting from high runtime performance. This is especially helpful for applications that need to start quickly but run for long periods.

Working of JIT compiler

After getting the bytecode, the JIT compiler transforms it into the native code. However, that bytecode may have a significant impact on the speed and performance of an application. To improve performance, the JIT compiler will, at runtime, compile suitable bytecode sequences into machine code. The code is typically sent to a processor, where the code instructions are carried out. Code that looks like it can be re-optimized is called “Hot.” With less chance of code being interpreted multiple times, there is less overhead, faster execution speeds. This is why most of the implementations of JVM use JIT compilers.

Work Flow of JIT

JDK provides a java compiler to compile the java source code into the bytecode. After that, JVM loads the .class file at runtime and transforms the bytecode into the binary code. Further, the machine code is used by the interpreter.

• The interpretation of Java bytecode reduces the performance of the native application. It is the reason to implement the JIT compiler. The JIT compiler increases the performance of the application by compiling the bytecode into machine code,
• It is enabled by default when a method is invoked. The JVM directly invokes the compiled code of the procedure without interpreting it. It does not require much memory usage.

Check out this article - Compile Time Polymorphism

Types of JIT Compilers in Java

The Just-In-Time (JIT) compiler plays a crucial role in optimizing Java applications by converting bytecode to native machine code at runtime. The JVM includes different types of JIT compilers to suit varying performance needs. Understanding the types of JIT compilers in Java—C1, C2, and Tiered Compilation—helps developers choose the best strategy for their application’s behavior and runtime profile.

Client Compiler (C1)

The C1 compiler, also called the client compiler, focuses on quick compilation and fast application startup. It applies lightweight optimizations and is ideal for desktop applications, command-line tools, or environments where responsiveness is more critical than long-term performance.

C1 prioritizes speed over complexity, compiling methods quickly to minimize startup delay. This makes it well-suited for small or short-lived Java programs that don’t benefit much from advanced optimizations.

Best for: GUI apps, development tools, and short-running processes.

Server Compiler (C2)

The C2 compiler, or server compiler, is designed for deep optimization of long-running applications. It performs sophisticated techniques like method inlining, loop unrolling, and escape analysis. While it takes more time to compile methods, the resulting code executes faster over time.

C2 is commonly used in Java web servers, enterprise systems, and backend services, where performance after startup is critical. The trade-off is slower initial execution, but significantly better throughput and long-term efficiency.

Best for: High-performance applications such as microservices, application servers, and big data platforms.

Tiered Compilation (C1 + C2)

Tiered compilation in the JVM combines the best of both C1 and C2. It starts with C1 for faster method compilation and quick startup, and then transitions to C2 as methods become "hot" (frequently called), optimizing them for better runtime performance.

This hybrid approach offers the best balance between startup time and execution efficiency, and is enabled by default in modern JVMs like HotSpot. It allows the JVM to profile methods while using C1 and use that data to guide C2’s advanced optimizations.

Best for: General-purpose applications, especially where both quick startup and sustained performance matter.

Key Differences Between JIT and AOT

Understanding the differences between JIT and AOT compilers in Java helps in selecting the right compilation approach for specific application needs.

Type 1: Compilation Time

JIT compiles methods during execution, allowing dynamic analysis and tuning. In contrast, AOT compiles ahead of time, so it cannot adapt to runtime behavior but offers faster launches, useful for quick-execution environments.

Type 2: Performance Optimization

JIT enables adaptive optimization, using real-time profiling to inline methods or optimize hot code paths. AOT uses fixed optimizations, which might not match runtime behavior, possibly leading to suboptimal performance.

Type 3: Startup Time and Portability

AOT-compiled applications launch quickly and are often used in embedded or low-latency systems. JIT, while slower at startup, allows Java to remain platform-independent and eventually achieves better long-term performance.

Which One to Use and When?

Use JIT compilation when developing long-running Java applications such as web servers, games, or batch processors—where runtime profiling and adaptive optimizations significantly enhance performance.

Use AOT compilation in resource-constrained or latency-sensitive environments, like mobile apps or embedded systems, where fast startup and smaller runtime environments are preferred. GraalVM supports AOT compilation for such scenarios, combining the flexibility of Java with the performance of native binaries.

Real-World Examples of JIT in Action

JVM Behavior During Repeated Method Calls

The JVM uses profiling to detect "hot methods"—those executed frequently. Initially, these methods run in interpreted mode. Once their usage crosses a threshold, the JIT compiler compiles them into native code, applying optimizations.

public class JITExample {
    public static void main(String[] args) {
        for (int i = 0; i < 1_000_000; i++) {
            compute(); // JIT will optimize this method over time
        }
    }
    static void compute() {
        int result = 10 * 5 + 3;
    }
}

You can also try this code with Online Java Compiler

Run Code

After many executions, compute() is compiled into optimized native code, making it faster on subsequent calls. This demonstrates how JIT optimization in Java improves performance during execution.

Performance Boost in Java Applications

JIT compilation reduces method call overhead, optimizes memory access patterns, and applies techniques like branch prediction. This results in faster loops, quicker method calls, and improved CPU cache usage.

Real-world benefits of JIT include:

• Web servers handling more concurrent requests with lower latency.
   
• Microservices scaling efficiently under heavy load.
   
• Game engines delivering smoother animations and faster logic processing.
   

These examples highlight Java performance improvement with JIT, making it an essential feature for modern application development.

Advantages and disadvantages of JIT

Advantages

• JIT needs less memory usage:
• JIT compilers run after a program starts.
• Code optimization can be performed while coding execution.
• Cade can be localised on the same page.
• It can use different levels of optimization.

Disadvantages:

• Startup takes up a lot of time.
• Heavy usage of cache memory.
• Increases the level of complexity in Java programs.

Frequently Asked Questions

What is the difference between JIT and JVM compilation?

A: The JVM directly calls the compiled code instead of interpreting it. If compiling did not require any processor time or memory usage, the speed native compiler would have been the same. JIT compilation requires processor time and memory usage.

Can JIT be disabled in Java?

Yes, JIT can be disabled using the JVM option -Xint, which forces the JVM to run in interpreted mode only, bypassing all JIT compilation.

How do I know if JIT is working in my JVM?

You can enable verbose JIT logging using the -XX:+PrintCompilation JVM flag. It shows when methods are compiled by the JIT during execution.

How much does the code cache consume?

A: The JIT compiler uses memory intelligently. When the code cache is initialised, it consumes relatively more minor memory. Space that was previously occupied by discarded methods is reclaimed and reused. The JIT compiler methods avoid exhausting the system memory and affecting the system memory and affecting the stability of the application or the operating system.

Conclusion

In this blog, we covered the topics mentioned below:

• What is the JIT compiler?
• Working of JIT compiler
• Work Flow of the JIT compiler
• Advantages and disadvantages of the JIT compiler.

If you want to explore more about JIT, you can visit this webpage, JIT Compiler.

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