JIT in Java

When we write a program in any programming language it requires converting that code in the machine-readable form because the machine only understands the binary language. According to the programming languages, compiler differs.

The compiler is a program that converts the high-level language to machine level code. The Java programming language uses the compiler named javac. It converts the high-level language code into machine code (bytecode).

What is JIT in Java?

In Java, JIT (Java-In-Time Compiler) is an integral part of the JVM. It optimizes the performance many times over the previous level. In other words, it is a long-running, computer-intensive program that provides the best performance environment. It optimizes the performance of the Java application at compile or run time. The JIT compilation is also known as dynamic compilation.

The JIT compilation includes two approaches AOT (Ahead-of-Time compilation) and interpretation to translate code into machine code.

• AOT compilation translates Java bytecode into native machine code before runtime.
• It was introduced as an experimental feature in Java 9 via JEP 295.
• The tool used for AOT in standard JVMs is jaotc, which leverages the Graal compiler.

Note: Ahead-of-Time (AOT) compilation is not a core part of the JVM, but it is a feature that can be used alongside it.

JVM Relationship

• The JVM still plays a crucial role even when AOT is used:
• It handles class loading, garbage collection, and standard library execution.

The following optimizations are done by the JIT compilers:

• Method In-lining
• Local Optimizations
• Control Flow Optimizations
• Constant Folding
• Dead Code Elimination
• Global Optimizations
• Heuristics for optimizing call sites

Advantages of JIT Compiler

• It requires less memory usages.
• The code optimization is done at run time.
• It uses different levels of optimization.
• It reduces the page faults.

Disadvantages of JIT Compiler

• It increases the complexity of the program.
• The program with less line of code does not take the benefit of the JIT compilation.
• It uses lots of cache memory.

Working of JIT Compiler

If the JIT compiler environment variable is properly set, the JVM reads the .class file (bytecode) for interpretation after that it passes to the JIT compiler for further process. After getting the bytecode, the JIT compiler transforms it into the native code (machine-readable code).

• Java Development Kit (JDK) provides the Java compiler (javac) to compile the Java source code into the bytecode (.class file). After that, JVM loads the .class file at runtime and transform the bytecode into the binary code (machine code). Further, the machine code is used by the interpreter.
• We know that the interpretation of Java bytecode reduces the performance of the native application. It is the reason to implement the JIT compiler. The JIT compiler accelerates the performance of the application by compiling the bytecode into native machine code.
• It is enabled by default when a method is invoked. The JVM directly invokes the compiled code of the method without interpreting it. It does not require much memory usage.

Therefore, the JIT compiler boosts the performance of the native application. We can understand the working of the JIT compiler with the help of the following flow chart.

The following figure shows the functional relationship of the JIT compiler with JRE and JVM.

Optimization Levels

It is also known as the optimization level. Each level provides a certain level of performance. JIT compiler provides the following level of optimization:

• Cold: It used during the startup of the large Java application. The goal is to achieve the best-compiled code speed.
• Warm: After the starting of the Java application, most of the methods compiled when they reach the invocation threshold.
• Hot: The methods that consume more than 1% are scheduled for hot compilation.
• Very Hot: The method scheduled for a very hot if they are not scorching but hot.
• Scorching: The methods that consume more than 5% are scheduled for scorching compilation.

The default and initial optimization levels are warm. We get better performance if the optimization level is hotter but it increases the cost in terms of memory and CPU.

At the higher optimization level, the virtual machine uses a thread called sampling. It identifies the methods that take a long time to execute. The higher optimization level includes the following techniques:

• Escape Analysis
• Partial Redundancy Elimination

The above techniques use more memory and CPU to improve the performance of the Java application. It increases the cost of compilation but compensates for performance.

Conclusion

Just-in-Time (JIT) compilation is a crucial component of the Java Virtual Machine (JVM) that significantly enhances the performance of Java applications. By transforming bytecode into optimized native machine code at runtime, the JIT compiler effectively eliminates the performance overhead of interpretation.

Its dynamic optimization capabilities, such as method inlining and dead code elimination, allow it to tailor the compiled code to the specific execution profile of an application, leading to superior performance for long-running programs.