Class.forName("Java.util.Date")是什么加载到内存的？求解释

啷个里个郞 发表于 2012-5-6 13:24
先到缓冲区中查找，如果缓冲区中已经存在该类的字节码，那么就可直接使用，
如果没有，那么通过如二楼说的 ...

也祝你成功

隋营营 发表于 2012-5-6 13:53
代码最能说明问题：
public class Testext {
    public static void main(String[] args) throws Exceptio ...

很详细，十分感谢

用Class.forName(java.util.Date);来返回字节码，返回有两种情况：

1、是字节码已经加载到了虚拟机中只要返回就行

2、是虚拟机中还没有字节码，用类加载器去加载，把加载进来的的字节码缓存到虚拟机中返回

代码最能说明问题：
public class Testext {
    public static void main(String[] args) throws Exception {
        Class.forName("java.util.Date");
    }
}
在Eclipse的Run Configurations中设置VM arguments：-verbose:class
运行程序输出如下：（很长……有点耐心，关键在末尾的输出语句！）

[Loaded java.lang.Object from shared objects file]
[Loaded java.io.Serializable from shared objects file]
[Loaded java.lang.Comparable from shared objects file]
[Loaded java.lang.CharSequence from shared objects file]
[Loaded java.lang.String from shared objects file]
[Loaded java.lang.reflect.GenericDeclaration from shared objects file]
[Loaded java.lang.reflect.Type from shared objects file]
[Loaded java.lang.reflect.AnnotatedElement from shared objects file]
[Loaded java.lang.Class from shared objects file]
[Loaded java.lang.Cloneable from shared objects file]
[Loaded java.lang.ClassLoader from shared objects file]
…………
[Loaded java.lang.StringBuilder from shared objects file]
[Loaded java.lang.StackTraceElement from shared objects file]
[Loaded java.nio.Buffer from shared objects file]
[Opened D:\Soft\Program\JAVA\JDK\jre\lib\rt.jar]
[Loaded sun.misc.PostVMInitHook from shared objects file]
[Loaded java.lang.Boolean from shared objects file]
[Loaded java.lang.Character from shared objects file]
[Loaded java.lang.Number from shared objects file]
[Loaded java.util.concurrent.atomic.AtomicInteger from shared objects file]
[Loaded java.lang.reflect.ReflectAccess from shared objects file]
[Loaded java.util.Arrays from shared objects file]
[Loaded java.nio.charset.Charset$3 from D:\Soft\Program\JAVA\JDK\jre\lib\rt.jar]
[Opened D:\Soft\Program\JAVA\JDK\jre\lib\charsets.jar]
[Loaded sun.nio.cs.AbstractCharsetProvider from D:\Soft\Program\JAVA\JDK\jre\lib\rt.jar]
[Loaded sun.nio.cs.ext.ExtendedCharsets from D:\Soft\Program\JAVA\JDK\jre\lib\charsets.jar]
[Loaded java.lang.Class$1 from shared objects file]
[Loaded sun.reflect.ReflectionFactory$1 from shared objects file]
[Loaded sun.reflect.NativeConstructorAccessorImpl from shared objects file]
[Loaded sun.reflect.DelegatingConstructorAccessorImpl from shared objects file]
[Loaded java.util.SortedMap from shared objects file]
[Loaded java.util.NavigableMap from shared objects file]
[Loaded java.util.TreeMap from shared objects file]
[Loaded sun.misc.ASCIICaseInsensitiveComparator from shared objects file]
[Loaded java.util.TreeMap$Entry from D:\Soft\Program\JAVA\JDK\jre\lib\rt.jar]
[Loaded sun.misc.VM from shared objects file]
………………
[Loaded java.io.FilePermission from shared objects file]
[Loaded java.io.FilePermission$1 from shared objects file]
[Loaded java.io.FilePermissionCollection from shared objects file]
[Loaded java.security.AllPermission from shared objects file]
[Loaded java.security.UnresolvedPermission from shared objects file]
[Loaded java.security.BasicPermissionCollection from shared objects file]
[Loaded sttt.Testext from file:/D:/DTemp/JAVAWorkspace/suiusoar/bin/] 这是我自己写的类
[Loaded java.lang.Void from shared objects file]
[Loaded java.util.Date from shared objects file] java.util.Date在这里被加载到内存
[Loaded sun.util.calendar.CalendarSystem from shared objects file]
[Loaded sun.util.calendar.AbstractCalendar from shared objects file]
[Loaded sun.util.calendar.BaseCalendar from shared objects file]
[Loaded sun.util.calendar.Gregorian from shared objects file]
[Loaded java.lang.Shutdown from shared objects file]
[Loaded java.lang.Shutdown$Lock from shared objects file]
看到了吗？其中有我们非常熟悉的类

这里涉及到了JAVA的ClassLoader的运行机制：
bootstrapclass loader：装载JDK最核心的类（如java.lang包中的类）以及其它的ClassLoader，其它的 ClassLoader再装载其它类
ExtClassLoader：装载目录jre\lib\ext目录下的类
AppClassLoader：装载我们自己写的类（这里是sttt.Testext ）

JDK在加载类时首先看这个类是否被上一层的ClassLoader加载过了，若已经加载，不会重新加载！

先到缓冲区中查找，如果缓冲区中已经存在该类的字节码，那么就可直接使用，
如果没有，那么通过如二楼说的那样。希望能帮助你，我们一起加油。

Class.forName("Java.util.Date")内部调用了ClassLoader的
方法。
"Java.util.Date"就是有效的二进制名称。

先到缓冲区中查找，如果已经加载了Date 对象的字节码，那么他就直接使用，
如果没有，那么他就加载Date字节码到内存.....
所以Class比较消耗内存