最近遇到一个这样的问题，项目是一个spring cloud的项目，一个主模块(记为mainMoudle)依赖了另一个子模块(记为subMoudle)。在开发过程中，在idea开发工具环境下是能正常运行的，但由于测试时，需要将模块打包，就将subMoudle工程打成了一个jar放在mainMoudle下，跑jar包时就发现不能运行了，控制台抛出了fileNotFoundException的异常信息。
通过查看subMoudle下的代码排查问题时，我发现是由于subMoudle在初始化时，需要加载mainMoudle中的配置文件。加载的代码是通过File类直接加载的，在开发环境时，运行时是直接将工程的资源文件编译到target的classes目录下的， 所以在开发环境下是可以正常运行的。而当项目打成了一个jar包时运行时,jar包中的资源文件不会再自动解压释放到目录中的，因为它已经编译好了，java也已经它成了class字节码文件了。所以再通过原来的File直接读取jar下的一个文件时是读取不到的，故问题就出现在这里。那么我们该如何去解决这个问题呢，当时为了方便我直接用的apache的commons-configuration包来解决的，将subMoudle的中对于读取配置文件的代码进行了替换，对于要读取配置文件的代码全改成了configuration的代码来读取，问题就解决了。
他们都说知道如何解决一个问题是一个初级程序员的该干的事，作为一个中高级程序员就必须得要了解其原理了，我觉得很有道理。于是我通过在idea下的断点进行了分析，找到了关键代码
static URL locateFromClasspath(String resourceName) {
URL url = null;
ClassLoader loader = Thread.currentThread().getContextClassLoader();
if (loader != null) {
url = loader.getResource(resourceName);
if (url != null) {
LOG.debug("Loading configuration from the context classpath (" + resourceName + ")");
}
}
if (url == null) {
url = ClassLoader.getSystemResource(resourceName);
if (url != null) {
LOG.debug("Loading configuration from the system classpath (" + resourceName + ")");
}
}
return url;
}首先它通过获取了当前线程的一个类加载器，通过加载器的getResouce方法去类加载器找到resourceName这个文件
loader.getResouce的代码属于JDK的代码，其getResouce这个方法代码为：
// -- Resource --
/**
* Finds the resource with the given name. A resource is some data
* (images, audio, text, etc) that can be accessed by class code in a way
* that is independent of the location of the code.
*
* <p> The name of a resource is a '<tt>/</tt>'-separated path name that
* identifies the resource.
*
* <p> This method will first search the parent class loader for the
* resource; if the parent is <tt>null</tt> the path of the class loader
* built-in to the virtual machine is searched. That failing, this method
* will invoke {@link #findResource(String)} to find the resource. </p>
*
* @apiNote When overriding this method it is recommended that an
* implementation ensures that any delegation is consistent with the {@link
* #getResources(java.lang.String) getResources(String)} method.
*
* @param name
* The resource name
*
* @return A <tt>URL</tt> object for reading the resource, or
* <tt>null</tt> if the resource could not be found or the invoker
* doesn't have adequate privileges to get the resource.
*
* @since 1.1
*/
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;
}jdk的开发者还为我们留下了注释，注意这段注释：
* <p> This method will first search the parent class loader for the
* resource; if the parent is <tt>null</tt> the path of the class loader
* built-in to the virtual machine is searched. That failing, this method
* will invoke {@link #findResource(String)} to find the resource. </p>通过代码和注释我们可以得知此代码会先去父节点的loader去加载资源文件，如果找不到，则会去BootstrapLoader中去找，如果还是找不到，才调用当前类的classLoader去找。这也就是我们有时说的所谓的双亲委派模型。
（双亲委派模型的工作过程为：如果一个类加载器收到了类加载的请求，它首先不会自己去尝试加载这个类，而是把这个请求委派给父类加载器去完成，每一个层次的加载器都是如此，因此所有的类加载请求都会传给顶层的启动类加载器，只有当父加载器反馈自己无法完成该加载请求（该加载器的搜索范围中没有找到对应的类）时，子加载器才会尝试自己去加载）
public InputStream getInputStream(URL url) throws ConfigurationException {
File file = ConfigurationUtils.fileFromURL(url);
if (file != null && file.isDirectory()) {
throw new ConfigurationException("Cannot load a configuration from a directory");
} else {
try {
return url.openStream();
} catch (Exception var4) {
throw new ConfigurationException("Unable to load the configuration from the URL " + url, var4);
}
}
}当资源被找到后，通过调用url的openStream()方法去获得此文件的输入流
因此，单纯地用File去去读取jar包的文件是不能的，因为！并不是文件资源定位符的格式 (jar中资源有其专门的URL形式： jar:<url>!/{entry} )。所以，如果jar包中的类源代码用File f=new File(相对路径);的形式，是不可能定位到文件资源的
为此我专门写了个测试代码：
import org.apache.commons.configuration.Configuration;
import org.apache.commons.configuration.ConfigurationException;
import org.apache.commons.configuration.PropertiesConfiguration;
import java.io.*;
import java.net.URL;
import java.util.Iterator;
public class ResourceReader {
private static final String subMoudlePropertiesFile = "sys.properties";//jar下的配置文件
private static final String innerPropertiesFile = "own.properties";//内部配置文件
public static void main(String[] args) throws InterruptedException {
loadJarFileByConfiguration();
Thread.sleep(1000);
loadLocalFile();
Thread.sleep(1000);
loadJarFileByResource();
Thread.sleep(1000);
loadJarFileByFile();
}
/**
* 通过File类去加载jar包的资源文件
*/
private static void loadJarFileByFile() {
System.out.println("----------loadJarFileByFile---- begin------------");
URL resource = ResourceReader.class.getClassLoader().getResource(subMoudlePropertiesFile);
String path = resource.toString();
System.out.println(path);
try {
File file = new File(path);
FileInputStream fileInputStream = new FileInputStream(file);
BufferedReader br = new BufferedReader(new InputStreamReader(fileInputStream));
String s = "";
while ((s = br.readLine()) != null)
System.out.println(s);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("----------loadJarFileByFile---- end------------\n\n");
}
/**
* 通过apache configuration包读取配置文件
*/
private static void loadJarFileByConfiguration() {
System.out.println("----------loadJarFileByConfiguration---- begin------------");
try {
Configuration configuration = new PropertiesConfiguration(subMoudlePropertiesFile);
Iterator<String> keys = configuration.getKeys();
while (keys.hasNext()) {
String next = keys.next();
System.out.println("key:" + next + "\tvalue:" + configuration.getString(next));
}
} catch (ConfigurationException e) {
e.printStackTrace();
}
System.out.println("----------loadJarFileByConfiguration---- end------------\n\n");
}
/**
* 通过类加载器去的getResource方法去读取
*/
private static void loadJarFileByResource() {
System.out.println("----------loadJarFileByResource---- begin------------");
URL resource = ResourceReader.class.getClassLoader().getResource(subMoudlePropertiesFile);
String path = resource.toString();
System.out.println(path);
try {
InputStream is = resource.openStream();
BufferedReader br = new BufferedReader(new InputStreamReader(is));
String s = "";
while ((s = br.readLine()) != null)
System.out.println(s);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("----------loadJarFileByResource---- end------------\n\n");
}
/**
* 读取当前工程中的配置文件
*/
private static void loadLocalFile() {
System.out.println("----------loadLocalFile---- begin------------");
String path = ResourceReader.class.getClassLoader().getResource(innerPropertiesFile).getPath();
System.out.println(path);
try {
FileReader fileReader = new FileReader(path);
BufferedReader bufferedReader = new BufferedReader(fileReader);
String strLine;
while ((strLine = bufferedReader.readLine()) != null) {
System.out.println("strLine:" + strLine);
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("----------loadLocalFile---- begin------------\n\n");
}
}子模块结构为：
sys.properties位于subMoudle的jar中
以上代码运行结果为:
----------loadJarFileByConfiguration---- begin------------
log4j:WARN No appenders could be found for logger (org.apache.commons.configuration.PropertiesConfiguration).
log4j:WARN Please initialize the log4j system properly.
key:username value:haiyangge
key:password value:haiyangge666
----------loadJarFileByConfiguration---- end------------
----------loadLocalFile---- begin------------
/E:/idea_space/spring_hello/target/classes/own.properties
strLine:db.username=9527
strLine:db.password=0839
----------loadLocalFile---- begin------------
----------loadJarFileByResource---- begin------------
jar:file:/E:/idea_space/spring_hello/libs/subMoudle-1.0-SNAPSHOT.jar!/sys.properties
username=haiyangge
password=haiyangge666
----------loadJarFileByResource---- end------------
----------loadJarFileByFile---- begin------------
java.io.FileNotFoundException: jar:file:\E:\idea_space\spring_hello\libs\subMoudle-1.0-SNAPSHOT.jar!\sys.properties (文件名、目录名或卷标语法不正确。)
jar:file:/E:/idea_space/spring_hello/libs/subMoudle-1.0-SNAPSHOT.jar!/sys.properties
----------loadJarFileByFile---- end------------
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at javafile.read.ResourceReader.loadJarFileByFile(ResourceReader.java:35)
at javafile.read.ResourceReader.main(ResourceReader.java:22)
从运行结果可以看出，通过file类去加载本项目中的资源文件是可以成功的，但加载jar下的资源文件是不可以的，因为jar!sys.properties不是文件资源定位符的格式，而是jar中的.
故加载jar包内的资源文件时，应该用classLoader的getResource方法去加载,获取到URL后，用openStream()方法打开流，不应该原生的file去加载。