Lazy log message evaluation in java

Question

In some instances, I am interested in deferring the creation of the log message, so it won't be created in the case the log level is not sufficient.

I do it by passing a callback to a logging function, which is called if the logging level is sufficient.

The problem is I can't pass a variable which is not effectively final, because of closure limitations Java has.

What is a cleaner way to do it?

Running code: http://tpcg.io/IspNlN

import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.function.Supplier;

public class StackOverflow
{
    private static final int LOG_LEVEL=2;
    private static final int TRACE_LEVEL=0;

    private static void trace(Supplier<String> messageCallback){
        if(TRACE_LEVEL>LOG_LEVEL) return;
        System.out.println(messageCallback.get());          
    }

    private static void foo(){      
        for(int i=0; i<10; i++){
            doSomething();
            int finalI=i; //I need it because of "Local variable parameterIndex defined in an enclosing scope must be final or effectively final"
            trace(()->"FLOW-1: "+getHugeString()+" for i:"+finalI);
        }
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        foo();
    }

    private static  void doSomething(){
        //some operation
    }

    private static String getHugeString(){          
        return "A huge string that took much time to create";
    }
}

Answer 1

You have three options mentioned in order of preference.

Option 1

Use a proper logging library.

Option 2 as pointed by @Glains

private static boolean isTraceEnabled()
{
    return TRACE_LEVEL <= LOG_LEVEL;
}

private static void trace(String msg)
{
    if (isTraceEnabled())
    {
        System.out.println(msg);
    }
}

private static void foo()
{
    for (int i = 0; i < 10; i++)
    {
        doSomething();
        if (isTraceEnabled())
        {
            trace("FLOW-1: " + getHugeString() + " for i:" + i);
        }
    }
}

Option 3 use message format as in jdk logger.

private static void trace(String format, Object... values)
{
    if (TRACE_LEVEL > LOG_LEVEL)
    {
        return;
    }
    for (int i = 0; i < values.length; i++)
    {
        Object value = values[i];
        if (value instanceof Supplier)
        {
            values[i] = ((Supplier) value).get();
        }
    }
    System.out.println(new MessageFormat(format).format(values));
}

invocation like

trace("FLOW-1: {0} for i: {1}", (Supplier) (StackOverflow::getHugeString), i);

Answer 2

Common logging frameworks offer methods like isDebugEnabled or isInfoEnabled to guard exactly against that case where Strings may take a longer time to be created.

if(LOG.isDebugEnabled()) {
    LOG.debug("Some objects toString method called: {}", someObject.toString());
}

Besides that, your method also works, but has the limitations that you pointed out already.