在Java中打印变量名称和变量值的方法

Question

String activityState = "resume";
DebugLog(activityState)

void DebugLog(String obj1) {}

How to make the DebugLog to print like this:

activityState : resume

I used to write many print statement as logs at many places while debugging. I will write statements like

System.out.println("activityState : " + activityState);

I want a method to print the variable name and variable value. In C++, it can be done like the below:

#define dbg(x) cout<< #x <<" --> " << x << endl ;

Is there any way to do this?

Thanks in advance.

Answer 1

There's no direct solution to get the variable name.

However, in a context where you have many fields and don't want to manually print their state, you can use reflection.

Here's a quick example:

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

    int i = 1;
    String s = "foo";

    @Override
    public String toString() {
        StringBuilder result = new StringBuilder();
        for (Field f: getClass().getDeclaredFields()) {
            try {
            result
            .append(f.getName())
            .append(" : ")
            .append(f.get(this))
            .append(System.getProperty("line.separator"));
            }
            catch (IllegalStateException ise) {
                result
                .append(f.getName())
                .append(" : ")
                .append("[cannot retrieve value]")
                .append(System.getProperty("line.separator"));
            }
            // nope
            catch (IllegalAccessException iae) {}
        }
        return result.toString();
    }
}

Output

i : 1
s : foo

Answer 2

You can use java Reflection to get the variable name and the value. Here's an example code;

public class Example{

  String activityState = "resume";

  public static void main(String[] args) {

       Example example = new Example();
       Class<?> c = example.getClass();
       Field field = c.getDeclaredField("activityState");
       System.out.println(field.getName());
       System.out.println(field.get(example));
  }    

}

Answer 3

Since this is for debugging you could use instrumentation with aspectj , your code remains clean from the the debugging output statements and you can waeve the aspect as needed.

Define a set(FieldPattern) point cut to catch all field assignements (join points)

public aspect TestAssignmentAspect {

    pointcut assigmentPointCut() : set(* *);

    after() : assigmentPointCut() {
        System.out.printf("%s = %s%n", thisJoinPoint.getSignature().getName(),
                String.valueOf(Arrays.toString(thisJoinPoint.getArgs())));
    }
}

Here is Test class

public class Test {

    public static String activityState = "stopped";

    public static void main(String[] args) {
        activityState = "start";

        doSomething();

        activityState = "pause";

        doSomeOtherthing();

        activityState = "resume";

        System.out.printf("the end!%n");
    }

    private static void doSomeOtherthing() {
        System.out.printf("doing some other thing...%n");
    }

    private static void doSomething() {
        System.out.printf("doing something...%n");
    }
}

If you run this example with the aspect weaved the output will be

activityState = [stopped]
activityState = [start]
doing something...
activityState = [pause]
doing some other thing...
activityState = [resume]
the end!

Explanation

pointcut assigmentPointCut() : set(* *);

set point cut to catch assignments, the point joins, to any variable with any name, could also in the example be

pointcut assigmentPointCut() : set(String activityState);

The advice , the desired behavior when the given point cut matches

after() : assigmentPointCut() { ... }

Informations about the point join can be accessed using the special reference thisJoinPoint .