In JAVA, some source codes are often controlled by compilation instead of compiled to bytecode files.

JAVA will optimize the code during compilation. If a line of code is never executed,
This line of code will not be compiled into bytecode files.
In the following example, flag is a value that can be determined at the compilation stage,
During javac compilation, System. out. println is not compiled into bytecode files.

public class T {    public  final static boolean flag = false;    public void test(){        if(flag){            System.out.println("print this line");        }    }}

Javap verbose shows that the test method has no commands corresponding to System. out. println.

public void test();  Code:   Stack=0, Locals=1, Args_size=1   0:    return  LineNumberTable:   line 10: 0  LocalVariableTable:   Start  Length  Slot  Name   Signature   0      1      0    this       LT;}

If you change the flag to true
Public final static boolean flag = true;
The javap verbose command shows that the test method corresponds to System. out. println In the bytecode.

public void test();  Code:   Stack=2, Locals=1, Args_size=1   0:    getstatic    #19; //Field java/lang/System.out:Ljava/io/PrintStream;   3:    ldc    #25; //String print this line   5:    invokevirtual    #27; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   8:    return  LineNumberTable:   line 8: 0   line 10: 8  LocalVariableTable:   Start  Length  Slot  Name   Signature   0      9      0    this       LT;}

This feature can be used in log output scenarios to control some log output code through a compilation process.
It does not appear in the bytecode. However, to adjust the log output level, you need to re-compile the source code.