What is inlining?

Norman Maurer explains at his blog JVM and JIT inline functionality like that

Inlining is a technique that will basically just "inline" one method in another and so get rid of a method invocation. The JIT automatically detects "hot" methods and try to inline them for you. A method is considered "hot" if it was executed more the X times, where X is a threshold that can be configured using a JVM flag when start up java (10000 is the default). This is needed as inlining all methods would do more harm then anything else, because of the enormous produced byte-code. Beside this the JIT may "revert" previous inlined code when an optimization turns out to be wrong at a later state. Remember the JIT stands for Just in Time and so optimize (which includes inlining but also other things) while execute your code.

Also with a warning

But even if the JVM consider a method to be "hot" it may not inline it. But why? One of the most likely reasons is that it is just to big to get inlined.

And you can find a very simple code example for inlining a Java code at Eva Andreasson's Java World Post. You can find the related part of post at below.

Many optimizations try to eliminate machine-level jump instructions (e.g., JMP for x86 architectures). A jump instruction changes the instruction pointer register and thereby transfers the execution flow. This is an expensive operation relative to other ASSEMBLY instructions, which is why it is a common target to reduce or eliminate. A very useful and well-known optimization that targets this is called inlining. Since jumping is expensive, it can be helpful to inline many frequent calls to small methods, with different entry addresses, into the calling function. The Java code in Listings 3 through 5 exemplifies the benefits of inlining.

Listing 3. Caller method

int whenToEvaluateZing(int y) {
   return daysLeft(y) + daysLeft(0) + daysLeft(y+1);
}

Listing 4. Called method

int daysLeft(int x){
   if (x == 0)
      return 0;
   else
      return x - 1;
}

Listing 5. Inlined method

int whenToEvaluateZing(int y){
   int temp = 0;

   if(y == 0) temp += 0; else temp += y - 1;
   if(0 == 0) temp += 0; else temp += 0 - 1;
   if(y+1 == 0) temp += 0; else temp += (y + 1) - 1;

   return temp; 
}

In Listings 3 through 5 the calling method makes three calls to a small method, which we assume for this example's sake is more beneficial to inline than to jump to three times.

It might not make much difference to inline a method that is called rarely, but inlining a so-called "hot" method that is frequently called could mean a huge difference in performance. Inlining also frequently makes way for further optimizations, as shown in Listing 6.

Listing 6. After inlining, more optimizations can be applied

int whenToEvaluateZing(int y){
   if(y == 0) return y;
   else if (y == -1) return y - 1;
   else return y + y - 1;
}