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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 something.java 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 JavaPerformanceTuning.com team
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