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I've heard these terms being used, but i cant seem to find a top level view of where they fit in all together in a Java framework. I know JIT is a compiling mechanism, but is it part of JVM? Whats a Hotspot? Is it some new type of VM?
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JVM compiles complete byte code to machine code. JIT compiles only the reusable byte code to machine code. JVM provides platform independence. JIT improves the performance of JVM.
Which Java Virtual Machine to choose, HotSpot or OpenJ9? Both are tunable open-source JVM implementations. HotSpot is a well-established JVM implementation initially developed by Sun Microsystems. OpenJ9, developed by IBM, is not as widespread in the industry but has gained popularity in recent years.
HotSpot, released as Java HotSpot Performance Engine, is a Java virtual machine for desktop and server computers, developed by Sun Microsystems and now maintained and distributed by Oracle Corporation. It features improved performance via methods such as just-in-time compilation and adaptive optimization.
The JRockit JVM Runs JIT Compilation In theory, the JIT comes into use whenever a Java method is called, and it compiles the bytecode of that method into native machine code, thereby compiling it “just in time” to execute.
JIT is "Just In Time" compiling, basically compiling on the fly.
Hotspot is the concept within the JVM where it only compiles the code that's actually being used. That is, the "hot" pieces of code being used over and over.
The JVM tracks usage, and when something becomes popular enough, it queues that code for compilation, while continuing to interpret the code.
When the JIT finishes, it swaps out the interpreted bits with the compiled bits.
This is why a JVM needs to "warm up" for benchmarking and such.
The -server and -client options of the Sun/Oracle JVM affect this behavior as to how aggressive they are when doing the JIT work.
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