我们都知道JAVA初始化一个类的时候可以用new 操作符来初始化,也可通过Class.forName的方式来得到一个Class类型的实例,然后通过这个Class类型的实例的newInstance来初始化.我们把前者叫做JAVA的静态加载,把后者叫做动态加载.后者在很多框架中经常使用,通过用property文件的方式指定类名称.如我们熟悉的在写JDBC代码的时候加载不同数据库的驱动.
静态加载的时候如果在运行环境中找不到要初始化的类,抛出的是NoClassDefFoundError,它在JAVA的异常体系中是一个Error,错误.
动态态加载的时候如果在运行环境中找不到要初始化的类,抛出的是ClassNotFoundException,它在JAVA的异常体系中是一个checked异常,在写代码的时候就需要catch.
下面用实例来演示:ReferencedCls是需要初始化的类,它在一个单独的jar中,它被StaticReferencingCls和DynamicReferencingCls分别以静态和动态的方式加载.
将prjT1做成一个jar包,t1.jar.将prjT2做成一个jar包,t2.jar.放在d:/temp文件夹.
JAVA代码如下:
[java]
package com.test1;
public class ReferencedCls {
private String str = "test value";
public String getStr() {
return str;
}
public void setStr(String str) {
this.str = str;
}
public ReferencedCls() {
System.out.println("Constructor: ReferencedCls");
}
}
package com.test2;
import com.test1.ReferencedCls;
public class StaticReferencingCls {
public static void main(String[] args) {
System.out.println("Started testing static loading...............");
ReferencedCls cls = new ReferencedCls();
System.out.println(cls.getStr());
}
}
package com.test2;
import com.test1.ReferencedCls;
public class DynamicReferencingCls {
public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
System.out.println("Started testing dynamic loading...............");
Class cls = Class.forName("com.test1.ReferencedCls");
ReferencedCls obj = (ReferencedCls)cls.newInstance();
System.out.println(obj.getStr());
}
}
以不同的方式运行:
1,在运行环境中能找到需要的类时,都正常运行
[java]
D:\>java -cp d:/temp/t1.jar;d:/temp/t2.jar com.test2.StaticReferencingCls
Started testing static loading...............
Constructor: ReferencedCls
test value
D:\>java -cp d:/temp/t1.jar;d:/temp/t2.jar com.test2.DynamicReferencingCls
Started testing dynamic loading...............
Constructor: ReferencedCls
test value
2,以静态方式加载,运行环境缺失需要的类,抛出NoClassDefFoundError
[java]
D:\>java -cp d:/temp/t2.jar com.test2.StaticReferencingCls
Started testing static loading...............
Exception in thread "main" java.lang.NoClassDefFoundError: com/test1/ReferencedCls
at com.test2.StaticReferencingCls.main(StaticReferencingCls.java:9)
Caused by: java.lang.ClassNotFoundException: com.test1.ReferencedCls
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
... 1 more
3,以动态方式加载,运行环境缺失需要的类,抛出ClassNotFoundException,同时在写代码的时候也需要catch这个异常或者声明这个异常.
[java]
D:\>java -cp d:/temp/t2.jar com.test2.DynamicReferencingCls
Started testing dynamic loading...............
Exception in thread "main" java.lang.ClassNotFoundException: com.test1.ReferencedCls
at java.net.URLClassLoader$1.run(Unknown Sour