Does the Java compiler optimization a method called once by removing the method?

Question

Would the java compiler (any mainstream compiler) take something like

void foo () {
    int x = 1;
    System.out.println(x);
}

void bar() {
    foo()
}

and optimize the foo method away if it is only called in bar resulting in bytecode that would also be generated by this:

void bar() {
    int x = 1;
    System.out.println(x);
}

Most likely not since java compilers don't really optimize. They leave that to the JVM — Sami Kuhmonen, Jul 22 '16 at 19:42
It had better not. `foo` is visible outside the class it is defined in. Maybe if `foo` were private. — bradimus, Jul 22 '16 at 19:43
similar to: http://stackoverflow.com/questions/7772864/would-java-inline-methods-during-optimization — Slawomir Jaranowski, Jul 22 '16 at 20:03

score 2 · Accepted Answer · answered Jul 22 '16 at 20:01

No it doesn't. Javac doesn't do much optimization beyond constant folding. In java the optimization happens at runtime with the JIT.

I created a short example to show how the method did not get inlined in the bytecode even though it was only called once. Private and public make no difference.

Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String Hello before method
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: invokestatic  #5                  // Method publicMethod:()V
      11: invokestatic  #6                  // Method privateMethod:()V
      14: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      17: ldc           #7                  // String Hello after method
      19: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      22: return

  public static void publicMethod();
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #8                  // String The public method
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return

  private static void privateMethod();
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #9                  // String The private method
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

That means that if it crashes, the stacktrace is the optimised version usually, because it gets optimised on runtime? — Fabian Röling, Feb 08 '19 at 17:28

Does the Java compiler optimization a method called once by removing the method?

1 Answers1