Why doesn’t the JVM compile everything (all methods in all classes) to machine code up front?

JIT compilation takes time

• JIT compilation is expensive — analyzing and optimizing methods takes CPU cycles.
• Compiling everything upfront would cause longer startup time and higher resource use.

JVM goal: Fast startup + optimize only when needed.

Most code is cold

Studies (including JVM benchmarks) show that 90%+ of methods are cold in many real-world applications.

Compiling everything would bloat code cache and impact performance (especially on memory-constrained devices).

If only half the methods become hot, then only those hot methods are JIT-compiled into machine code and stored in the code cache.

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But what does “loaded in memory” really mean?

Class loading (via the ClassLoader):

• When a class is loaded, the entire bytecode of the class is read into memory (usually into Metaspace).
• This includes:
  • All method bytecode
  • Field info
  • Constant pool
  • Annotations, metadata, etc.

So yes, the whole class bytecode is loaded into memory — even if only some methods are used.

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JIT Compilation (runtime, method-based):

• The JVM only JIT-compiles individual hot methods, not the whole class.
• These hot methods are translated to machine code and stored in the code cache.
• Cold methods remain in memory as bytecode, and are interpreted when/if called.

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Important clarification:

The entire class (metadata + bytecode) is always loaded into memory after Class.forName() or any reference.

But:

Only selected hot methods are converted to machine code and loaded into the JVM code cache.

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Think of it like this:

Example Class:

javaCopyEditpublic class BigClass {
    public void methodA() { ... }  // called 1M times → HOT
    public void methodB() { ... }  // never called   → COLD
    public void methodC() { ... }  // called 2 times → COLD
}

What’s in memory:

Component	In memory?	Where stored
Bytecode (A, B, C)	Yes	JVM Metaspace
Machine code for methodA()	Yes	Code Cache
Machine code for methodB()	No	Not compiled
Machine code for methodC()	No	Not compiled