关于进口声明

Question

I want to know where can I check how many classes have been loaded by the class loader. I want to know, whether import statement in the class with * mark will load those many classes in the perm area of heap? OR only when we declare reference of the specific type then following two classes will be loaded?
Ex.

Map m = new HashMap(); //only these two classes loaded???

what if we write import statement like

java.util.*;

will class loader loads all classes under java.util package? Is there any class unloading process in java?

Answer 1

No, import has nothing to do with class loading. It is just syntax sugar. The following two programs will produce exactly the same byte code:

import java.util.Map;
import java.util.HashMap;
import java.util.List;
import java.util.ArrayList;
import java.util.prefs.*;

public class Foo
{
    private Map map = new HashMap ();
}

and

public class Foo
{
    private java.util.Map map = new java.util.HashMap ();
}

The time when class is loaded is up to JVM decision. Java specification only guarantees that class will be loaded and initialized before first use.

For example one JVM may decide to load all classes directly or indirectly referred by main class recursively before application startup. Such behavior could be handy for real-time applications that need to avoid non-deterministic pauses in program execution caused by class loading. Another JVM may defer class loading until the class is actually used and unload class immediately after it is not used anymore. This may be handy in embedded applications that are concerned by memory footprint.

Answer 2

import ed classes (or wildcards) are not actually loaded by the class loader. An import statement is merely an "alias" statement (or something like an "implicit prefix" statement in case you import wildcards).

Answer 3

这个工具VisualVM将帮助您。

Answer 4

If you want to see the classes that are loaded at runtime try this:

java -verbose:class

You should have a look for Question Is there a way to get which classes a ClassLoader has loaded?