Will compilers optimize out unused private methods?

Question

Does javac leave out private methods if they are inlined? For example

public class Test {
  public static void main(String[] args){
    System.out.println(sayHello());
  }
  private static String sayHello(){
    return "Hello World";
  }
}

But what if this method is then accessed by reflection?

 System.out.println(Test.class.getMethod("sayHello").invoke(null));

How does the compiler know whether to inline the method and remove it or not?

What makes you think this method is being inlined by the compiler? It does no inlining. Inlining might happen at runtime, in the JIT compiler. — JB Nizet, Oct 21 '17 at 15:32
You can try it out for yourself by disassembling the compiled class using `javap`. It will most likely not discard the method. — Jesper, Oct 21 '17 at 15:33
*"Does `javac` leave out private methods ..."* - No. Not under any circumstances. And `javac` doesn't do inlining anyway. (You can check both of these things for yourself using `javap`.) — Stephen C, Oct 21 '17 at 15:37

gil.fernandes · Accepted Answer · 2017-10-21T16:04:26.383

I have followed user Jesper's suggestion and used javap on this code:

package stackoverflow;

public class CompilerOptimize {
    public static void main(String[] args){
        System.out.println(sayHello());
    }

    private static String sayHello(){
        return "Hello World";
    }

    private static void bananas() {
        System.out.println("Monkey!");
    }
}

Method sayHello is used and bananas is not.

The result of running javap with the -p parameter on the compiled class with Java 8 is:

public class stackoverflow.CompilerOptimize {
  public stackoverflow.CompilerOptimize();
    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: invokestatic  #3                  // Method sayHello:()Ljava/lang/String;
       6: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       9: return

  private static java.lang.String sayHello();
    Code:
       0: ldc           #5                  // String Hello World
       2: areturn

  private static void bananas();
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #6                  // String Monkey!
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

The method bananas is mentioned in the byte code comments (sayHello is there too).

Will compilers optimize out unused private methods?

Not really.

@JB Nizet: shame on me. Many thanks for pointing out that the -p parameter was missing. You deserve some credit. — gil.fernandes, Oct 21 '17 at 16:04
Thanks for your answer. I never realized that javac didn't do any optimizing! Seems like missed opportunity though — vikarjramun, Oct 21 '17 at 19:58
Seems Android's Java-op-code optimizations work better by far, I mean, even always false if-block's code is removed (beside unused methods being removed). — Top-Master, May 27 '22 at 08:02

Will compilers optimize out unused private methods?

1 Answers1