带有子类的 Java 条件编译

Question

My apology if this was already asked somewhere else but I couldn't find it.

Basically, I would like to know whether a Java class with conditional compilation (for the purpose of removing all traces of a snippet of code in class files) will work on its subclasses.

For example, let's have class A and its subclass B implemented as follows:

public class A {
    protected static final boolean ASSERTS = false;

    protected A() {
        if (ASSERTS) {
            System.out.println("A: ASSERTS enabled.");
        }
}

public class B extends A {
    protected B() {
        if (ASSERTS) {
            System.out.println("B: ASSERTS enabled.");
        }
}

It's clear that nothing will be printed out. But, will their class files include the compiled if-condition?

Many thanks in advance!

Answer 1

Just compiled the class in IntelliJ, then decompiled the .class file to see if it contained the statement(s). It did not.

I suspect this is compiler dependent (ie the Java Language Specification makes no statement about whether or not these things must be included, so they are free to choose), and that you could probably find a compiler which would include these statements however. I was using Corretto Java 11.

Answer 2

No it will not.

You should try creating your .class files and then de-compile it using a decompiler or simply go to javadecompilers.com and upload your class file to confirm your answer.

Answer 3

Thanks for all suggestions! As suggested by @shivam-puri, I haved tried decompiled my class files. (Fyi, I'm using JDK1.8.0_212 and javadecompilers.com .)

Apparently, there is no trace of if-condition in the subclass B . That is, conditional compilation works on subclasses too.

Answer 4

if make sources based on your code

public class A {
protected static final boolean ASSERTS = false;

protected A() {
    if (ASSERTS) {
        System.out.println("A: ASSERTS enabled.");
    }
}

public static class B extends A {
  protected B() {
      if (ASSERTS) {
          System.out.println("B: ASSERTS enabled.");
        }
      }
    }
}

and compile with javac, then with FALSE we have byte-code

  public class A {
    protected static final boolean ASSERTS;
    protected A();
    Code:
       0: aload_0
       1: invokespecial #2                  // Method java/lang/Object."<init>":()V
       4: return
  }

with TRUE we get compiled byte-code below

public class A {
  protected static final boolean ASSERTS;

  protected A();
    Code:
       0: aload_0
       1: invokespecial #2                  // Method java/lang/Object."<init>":()V
       4: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
       7: ldc           #4                  // String A: ASSERTS enabled.
       9: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      12: return
}

so that on javac level such optimization works but for guaranty it is better to remove code parts on level of sources before compilation for instance with preprocessor

Answer 5

Java 不支持像 c、c++ 这样的宏您可以在运行时使用标志来启用或禁用断言，但不能使用宏