java依据classpath进行资源查找
一般在查找资源时，可根据本地路径和网络路径来定位具体的资源文件。其中本地路径又可分为绝对路径和相对路径。Java在查找资源时的最佳实践是通过相对路径来确定资源，java的类装载器ClassLoader内部包装了相对路径到绝对路径的转化实现。对于任意一个自定义类，可通过其Class类的getResource()方法获取到URL形式的资源绝对路径。
在具体的实践过程中，碰到一个小问题。测试用例如下：
工程结构：
Java-learning
|---- src/main/java
|---- src/test/java
|---- src/main/resources/META-INF/spring
|---- src/main/ resources
|---- 引用的jar包和maven的pom.xml等
测试代码包路径：
src/main/java
|---- com.lee.java.learning
|---- compiler
|---- Test.java
|---- AnClass.java
测试代码：
AnClass.java
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package com.lee.java.learning;
public class AnClass {
public static void main(String[] args) {
}
}
Test.java
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package com.lee.java.learning.compiler;
import java.net.URL;
public class Test {
public static void main(String[] args) {
Test test = new Test();
test.test();
}
public void test() {
URL root = this.getClass().getResource("/");
URL current1 = this.getClass().getResource("");
URL current2 = this.getClass().getResource(".");
URL parent = this.getClass().getResource("..");
URL self1 = this.getClass().getResource("Test.class");
URL self2 = this.getClass().getResource("./Test.class");
URL brother = this.getClass().getResource("../AnClass.class");
System.out.println("root = "+root);
System.out.println("current1 = "+current1);
System.out.println("current2 = "+current2);
System.out.println("parent = "+parent);
System.out.println("self1 = "+self1);
System.out.println("self2 = "+self2);
System.out.println("brother = "+brother);
}
}
测试输出结果：
root = file:/D:/Java/workspace_1/java-learning/target/test-classes/
current1 = file:/D:/Java/workspace_1/java-learning/target/test-classes/com/lee/java/learning/compiler/
current2 = file:/D:/Java/workspace_1/java-learning/target/test-classes/com/lee/java/learning/compiler/
parent = file:/D:/Java/workspace_1/java-learning/target/test-classes/com/lee/java/learning/
self1 = file:/D:/Java/workspace_1/java-learning/target/classes/com/lee/java/learning/compiler/Test.class
self2 = file:/D:/Java/workspace_1/java-learning/target/classes/com/lee/java/learning/compiler/Test.class
brother = file:/D:/Java/workspace_1/java-learning/target/classes/com/lee/java/learning/AnClass.class
分析应该是classpath的问题。eclipse下调试程序，观察命令行参数：
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E:\java6\bin\javaw.exe -agentlib:jdwp=transport=dt_socket,suspend=y,address=localhost:4584 -Dfile.encoding=UTF-8 -classpath D:\java\eclipse\my_workspace\java-learning\target\test-classes;D:\java\eclipse\my_workspace\java-learning\target\classes;E:\working\maven_lib\commons-logging\commons-logging\1.1.1\commons-logging-1.1.1.jar;E:\working\maven_lib\junit\junit\4.7\junit-4.7.jar;E:\working\maven_lib\org\springframework\spring\2.5.6\spring-2.5.6.jar com.sdo.lee.java.learning.compiler.Test
再查看了一下工程的.classpath文件：
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<?xml version="1.0" encoding="UTF-8"?>
<classpath>
<classpathentry kind="output" path="target/classes"/>
<classpathentry including="**/*.java" kind="src" path="src/main/java"/>
<classpathentry including="**/*.java" kind="src" output="target/test-classes" path="src/test/java"/>
<classpathentry excluding="**/*.java" kind="src" path="src/main/resources/META-INF/spring"/>
<classpathentry excluding="META-INF/spring/|**/*.java" kind="src" path="src/main/resources"/>
<classpathentry kind="var" path="M2_REPO/commons-logging/commons-logging/1.1.1/commons-logging-1.1.1.jar"/>
<classpathentry kind="var" path="M2_REPO/junit/junit/4.7/junit-4.7.jar"/>
<classpathentry kind="var" path="M2_REPO/org/springframework/spring/2.5.6/spring-2.5.6.jar"/>
<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.6"/>
</classpath>
发现更改上面的其中一条<classpathentry>如下，即可得到预期结果。
<classpathentry including="**/*.java" kind="src" output="target/classes" path="src/main/java"/>
同时，更改后的eclipse命令行参数如下：
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E:\java6\bin\javaw.exe -agentlib:jdwp=transport=dt_socket,suspend=y,address=localhost:4785 -Dfile.encoding=UTF-8 -classpath D:\java\eclipse\my_workspace\java-learning\target\classes;D:\java\eclipse\my_workspace\java-learning\target\test-classes;E:\working\maven_lib\commons-logging\commons-logging\1.1.1\commons-logging-1.1.1.jar;E:\working\maven_lib\junit\junit\4.7\junit-4.7.jar;E:\working\maven_lib\org\springframework\spring\2.5.6\spring-2.5.6.jar com.sdo.lee.java.learning.compiler.Test
eclipse在运行时是如何根据.classpath文件来添加命令行参数的-classpath属性不得而知。
(初步实践发现：eclipse可能是逐条读取classpathentry，一旦发现指定output属性，即将其path属性指定的路径添加到命令行参数的-classpath属性，并在最后将kind="output"指定的的默认path属性路径也添加到-classpath属性中。至于是否正确，有待验证。)
对于相对路径的解析('/'，'.'，'..'，''等)，java的源代码如下：
Class.class
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public java.net.URL getResource(String name) {
name = resolveName(name);
ClassLoader cl = getClassLoader0();
if (cl==null) {
// A system class.
return ClassLoader.getSystemResource(name);
}
return cl.getResource(name);
}
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private String resolveName(String name) {
if (name == null) {
return name;
}
if (!name.startsWith("/")) {
Class c = this;
while (c.isArray()) {
c = c.getComponentType();
}
String baseName = c.getName();
int index = baseName.lastIndexOf('.');
if (index != -1) {
name = baseName.substring(0, index).replace('.', '/')
+"/"+name;
}
} else {
name = name.substring(1);
}
return name;
}
ClassLoader.class
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public URL getResource(String name) {
URL url;
if (parent != null) {
url = parent.getResource(name);
} else {
url = getBootstrapResource(name);
}
if (url == null) {
url = findResource(name);
}
return url;
}
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public static URL getSystemResource(String name) {
ClassLoader system = getSystemClassLoader();
if (system == null) {
return getBootstrapResource(name);
}
return system.getResource(name);
}
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protected URL findResource(String name) {
return null;
}
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private static URL getBootstrapResource(String name) {
try {
// If this is a known JRE resource, ensure that its bundle is
// downloaded. If it isn't known, we just ignore the download
// failure and check to see if we can find the resource anyway
// (which is possible if the boot class path has been modified).
sun.jkernel.DownloadManager.getBootClassPathEntryForResource(name);
} catch (NoClassDefFoundError e) {
// This happens while Java itself is being compiled; DownloadManager
// isn't accessible when this code is first invoked. It isn't an
// issue, as if we can't find DownloadManager, we can safely assume
// that additional code is not available for download.
}
URLClassPath ucp = getBootstrapClassPath();
Resource res = ucp.getResource(name);
return res != null ? res.getURL() : null;
}
从上面的源代码可以看到，Class类的getResource方法先是进行一个简单的路径解析，然后将其委托给其装载器去处理，装载器则是委托其父装载器处理，一直到bootstrap装载器。bootstrap装载器是非java实现的，sun没有开源，具体如何实现不得可知，但有一点可以肯定的是根据classpath来进行装载。java的官方文档说明中讲到：classloader装载类时的优先级(-jar命令参数 > -classpath命令参数 > CLASSPATH环境变量 > 默认的'.'指定的当前路径)。详细可参考官方文档How Classes are Found一文。
在相对路径解析时，我们发现所有的相对路径是以'/'指定的.class文件根目录来进行解析。观察上面的eclipse命令行参数猜想：类装载器默认将-classpath参数指定的第一个目录作为'/'指定的根目录，即getResource('/')返回的结果，而一旦/后面跟有具体的文件时(如'./Test.class')，则会顺序遍历classpath参数指定的所有路径，找到相应的Test.class文件，返回其绝对路径的URL，如有多个，则返回第一个找到的Test.class文件的路径。
为求证，在纯命令行环境下进行实践，实践结果与猜想的一致。
总结：对于这种采用构建工具进行构建的项目，往往指定有多个.class文件输出目录(对于只有一个.class文件输出目录时，不存在本文开始处描述的问题)，因此在采用getResource(String name)之类的涉及到相对路径的方法调用，要稍加注意，有时候结果可能与预期出现偏差。
