Java类加载机制（一）：类加载器及其对应的加载路径

加载机制（一）：类加载器及其对应的加载路径"/>

Java类加载机制（一）：类加载器及其对应的加载路径

在讨论这个问题之前，我们先看看Java的两个重要的环境变量：Path与CLASSPATH

Path与CLASSPATH

Windows环境变量中的Path是用来指明Windows的可执行文件的路径。在安装JDK时，我们通常需要在Path的环境变量中添加java的可执行文件路径%JAVA_HOME%\bin，这样我们就可以在命令行中直接使用java、javac等命令，而无需添加这些可执行文件所在的路径。

CLASSPATH环境变量用于指定加载class文件时的搜索路径。会通过下面的实例说明CLASSPATH的意义。

Java的3种类加载器及其对应的加载路径

了解Java类加载机制应该就会知道，Java提供了3中类加载器：启动类加载器、扩展类加载器和应用程序类加载器。它们分别负责加载不同路径下的类库。在eclipse中通过下面的程序来打印出它们各自的加载路径：

package com.curry.classload;import java.URL;
import java.URLClassLoader;public class ClassLoaderTest {public static void main(String[] args){System.out.println("BootstrapClassLoader 的加载路径: ");//String[] bootUrls = System.getProperty("sun.boot.class.path").split(";");URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();for(URL url : urls)System.out.println(url);System.out.println("----------------------------");//取得扩展类加载器URLClassLoader extClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader().getParent();System.out.println(extClassLoader);System.out.println("扩展类加载器 的加载路径: ");urls = extClassLoader.getURLs();for(URL url : urls)System.out.println(url);System.out.println("----------------------------");//取得应用(系统)类加载器URLClassLoader appClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();System.out.println(appClassLoader);System.out.println("应用(系统)类加载器 的加载路径: ");urls = appClassLoader.getURLs();for(URL url : urls)System.out.println(url);System.out.println("----------------------------");	}}

得到的打印结果为：

BootstrapClassLoader 的加载路径: 
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/resources.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/rt.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/sunrsasign.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/jsse.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/jce.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/charsets.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/jfr.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/classes
----------------------------
sun.misc.Launcher$ExtClassLoader@15db9742
扩展类加载器 的加载路径: 
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/access-bridge-64.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/cldrdata.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/dnsns.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/jaccess.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/jfxrt.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/localedata.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/nashorn.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunec.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunjce_provider.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunmscapi.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunpkcs11.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/zipfs.jar
----------------------------
sun.misc.Launcher$AppClassLoader@73d16e93
应用(系统)类加载器 的加载路径: 
file:/C:/Users/Welcome/Documents/Eclipse/JVMInPractice/bin/
----------------------------

可以看出：启动类加载器的加载路径为%JAVA_HOME/jre，扩展类加载器的加载路径为%JAVA_HOME/jre/lib/ext，而应用程序类加载器的加载路径为该eclipse工程的类库。如果直接通过命令行编译并执行ClassLoaderTest.java文件，打印结果如下：

BootstrapClassLoader's load path:
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/resources.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/rt.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/sunrsasign.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/jsse.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/jce.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/charsets.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/jfr.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/classes
----------------------------
sun.misc.Launcher$ExtClassLoader@15db9742
extClassLoader's load path:
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/access-bridge-64.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/cldrdata.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/dnsns.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/jaccess.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/jfxrt.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/localedata.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/nashorn.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunec.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunjce_provider.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunmscapi.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/sunpkcs11.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/jre/lib/ext/zipfs.jar
----------------------------
sun.misc.Launcher$AppClassLoader@73d16e93
application classloader's load path:
file:/C:/Users/Welcome/Desktop/java/thought_oN_JVM/
file:/C:/Program%20Files/Java/jdk1.8.0_121/lib/dt.jar
file:/C:/Program%20Files/Java/jdk1.8.0_121/lib/tools.jar
----------------------------

可以发现：

（1）启动类加载器和扩展类加载器的加载路径没有发生变化。启动类加载器加载JAVA_HOME/jre下的核心类库，扩展类加载器加载JAVA_HOME/jre/lib/ext下的扩展类库

（2）应用程序类加载器的加载路径发生变化，这是因为eclipse会将其工程的CLASSPATH设置为这个工程专属的类库路径，而直接在命令行中编译执行时，应用程序类加载器的加载路径就是环境变量CLASSPATH的值。因此可以看出，java提供的应用程序类加载器负责的是CLASSPATH下类库的加载，如果用户程序没有自定义自己的类加载器，那么用户类一般情况下就是由应用程序类加载器加载。

参考文献