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<p><FONT class=normalfont><B><font color=blue>关于Java的Classpath详解</font></B></FONT><BR><FONT class=smallfont color=#ff9900>2004-04-23 15:18 pm</FONT><BR><FONT class=normalfont>作者:作者<br>来自:Linux知识宝库<br>联系方式:无名<br><br> Java 的新入门者对classpath往往比较困惑,为何在开发环境中能运行的东东出去就不好,或在外面运行的东东挺溜的进了开发环境就死菜。<br>
java的优点就是他是一个自动支持网络功能的东东,举一个简单例子来将<br>
<br>
比方有一个Application用到了java.lang.String(事实上要是有一个程序没用到它,太难以想象了)和com.shinechina.MyClass(我自己写的)这两个类。<br>
<br>
版本1:<br>
<br>
package com.shinechina;<br>
public class MyClass<br>
{<br>
public static void main(String args[])<br>
{<br>
String str = "版本1: hello, welcome to chinaasp java 技术";<br>
System.out.println(str);<br>
}<br>
}<br>
<br>
<br>
版本2:<br>
<br>
package com.shinechina;<br>
public class MyClass<br>
{<br>
public static void main(String args[])<br>
{<br>
String str = "版本2:hello, welcome to chinaasp java 技术";<br>
System.out.println(str);<br>
}<br>
}<br>
<br>
现在我要运行它了<br>
<br>
java com.shinechina.MyClass<br>
<br>
现在问题来了<br>
<br>
Java怎么知道到何处找呢, 他到底该怎么办<br>
<br>
假定版本一在 c:mylibcomshinechinaMyClass.java<br>
<br>
假定版本二在 d:mylibcomshinechinaMyClass.java<br>
<br>
注意:不要忘了编译成class文件吆, java文件是不能执行的<br>
<br>
cd c:mylibcomshinechina<br>
javac *.java<br>
cd d:mylibcomshinechina<br>
javac *.java<br>
<br>
<br>
有了:<br>
<br>
他先到机器的CLASSPATH去找,假设是windows系统 c:mylib;d:mylib;d:jdk1.2.2lib
t.jar;<br>
或unix系统 /home/local/:/usr/local/:/usr/local/jdk1.2.2/lib/rt.jar。好, c:mylib下他找到了com目录,有戏, 又往下找又有shinechina目录,目录下找到了MyClass.class, Ok 就是他了,至于d:mylib我的版本2的MyClass他就不理会了,至于String是在rt.jar压缩包中待会再讲他<br>
<br>
执行的结果就是<br>
<br>
版本1: hello, welcome to chinaasp java 技术<br>
<br>
反过来d盘的路径在c:之前,执行的结果就是<br>
<br>
版本2: hello, welcome to chinaasp java 技术<br>
<br>
依次类推, 只有在类路径上第一个找到的才会执行,你要控制他执行那一个须如此:<br>
<br>
java -classpath d:mylib;%classpath% com.shinechina.MyClass<br>
<br>
执行的结果就是<br>
<br>
版本2: hello, welcome to chinaasp java 技术<br>
<br>
开发环境中的执行就是这样,总有一个地方指定类路径的。<br>
<br>
注意,开发环境往往不去读系统路径, 这也就是在外面运行的东东挺溜的进了开发环境就死菜的原因喽。<br>
<br>
比如说jbuilder, jdevoloper从菜单project->project properties...->paths标签->Add...->添加定义好的library点ok或者new->输入name和classpath->ok。这个东东就被添加到了classpath中(运行时可在message view中看到)<br>
<br>
再比如说visualAge for java , 点中你要运行的类->鼠标右击选中特性->类路径标签1。->编辑->全选,好,开发环境中的所有类都能找到了。<br>
<br>
2、立即计算(只选中你需要的类, 如果有用class.forName之类通过程序显式装载的类的话,需要自己从编辑中手工家入了, 开发环境每能力帮你算出来了,如果内存没有问题的话,直接编辑->全选更方便吆)<br>
<br>
3、如果你的类并不想引入开发环境的话,点中添加额外路径,编辑(下面的一个), 我不喜欢这样, 就不详细介绍了<br>
<br>
高手的话,直接用jdk, 那么只好麻烦你要么加到系统classpath中(unix下为$CLASSPATH), 要么编译运行时显式指明classpath<br>
<br>
下面介绍一个初学者最容易犯的错误<br>
<br>
还以上节介绍的例子<br>
<br>
假定版本一在 c:mylibcomshinechinaMyClass.java<br>
<br>
编译后c:mylibcomshinechinaMyClass.class<br>
<br>
怎么指定classpath呢<br>
<br>
常见毛病<br>
<br>
1、c:mylibcomshinechina<br>
<br>
2、c:mylibcom<br>
<br>
正确<br>
<br>
1、c:mylib<br>
<br>
2、c:mylib<br>
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