Java Performance Tuning

Java(TM) - see bottom of page

|home |services |training |newsletter |tuning tips |tool reports |articles |resources |about us |site map |contact us |
Tools: | GC log analysers| Multi-tenancy tools| Books| SizeOf| Thread analysers|

Our valued sponsors who help make this site possible
JProfiler: Get rid of your performance problems and memory leaks! 

Question of the month: cost of import statements June 25th, 2003

Get rid of your performance problems and memory leaks!

Java Performance Training Courses
COURSES AVAILABLE NOW. We can provide training courses to handle all your Java performance needs

Java Performance Tuning, 2nd ed
The classic and most comprehensive book on tuning Java

Java Performance Tuning Newsletter
Your source of Java performance news. Subscribe now!
Enter email:

Get rid of your performance problems and memory leaks!

Back to newsletter 031 contents

Does using * in an import statement affect performance?

This question has come up a surprising number of times in various forums. The question relates to the import directive, as in

import java.util.*;

The short answer is: no, there is no affect on runtime performance.

The import directive is a compiler directive. The Java source to bytecode compiler reads the Java source file (i.e. a file) and converts that source to one or more bytecode files (.class files).

Most Java source code in the Java source file is converted into Java bytecodes of one sort or another. But the import directive is not. The import directive specifically tells the compiler to do something. The import directive tells the compiler that when it comes across a class or interface name in the source file, e.g. HashMap, then if it cannot find the class file for that name, it should use the import statement to help it look it up. This way, you don't have to always use fully qualified class names, e.g. java.util.HashMap.

The import directives themselves do not get put into the compiled bytecode files. They are no longer of any use, as the compiler compiles the fully qualified name into the .class file.

For example, suppose the source file contains

import java.util.*;
   HashMap map;

Then the compiler gets to the HashMap map variable declaration, tries to find a class called HashMap in the default package (i.e. no package name), fails, and uses the import statement to see if there is a java.util.HashMap class. This is found, and a reference to java.util.HashMap is inserted into the .class file. After compilation is completed, there is no use for the import directive, so it is not present at all in the compiled bytecode. Consequently, the import directive cannot affect runtime code execution in any way.

However, it is worth noting that the directive does affect compilation time. Since the directive tells the compiler how to do a second lookup to find classes, obviously this means that more time is spent looking up class and interface names compared to if one lookup sufficed.

In fact, there are two forms of the import directive: with and without the wildcard '*', e.g.

import java.util.*;
import java.util.HashMap;

Without the wildcard, the directive tells the compiler to look for one specific file in the classpath. With the wildcard, the directive is telling the compiler to look for the named package, and to search in that package for possible matches everytime any name needs to be matched. This latter version is probably going to be more costly (i.e. take longer) for the compiler than the former.

Also, depending on which compiler you use, and which settings, the compiler may re-compile all of the classes in the wildcard package, which could really make things take longer.

Finally, many people find that using imports with wildcards makes the source much less readable, since the reader also needs to figure out which package a particular class comes from, rather than just looking it up in the import statement.

So the full answer is

The team

Back to newsletter 031 contents

Last Updated: 2017-10-01
Copyright © 2000-2017 All Rights Reserved.
All trademarks and registered trademarks appearing on are the property of their respective owners.
Java is a trademark or registered trademark of Oracle Corporation in the United States and other countries. is not connected to Oracle Corporation and is not sponsored by Oracle Corporation.
RSS Feed:
Trouble with this page? Please contact us