用于从Java中的类路径加载资源的URL [英] URL to load resources from the classpath in Java
问题描述
在Java中,您可以使用相同的API但使用不同的URL协议加载所有类型的资源:
In Java, you can load all kinds of resources using the same API but with different URL protocols:
file:///tmp.txt
http://127.0.0.1:8080/a.properties
jar:http://www.foo.com/bar/baz.jar!/COM/foo/Quux.class
这很好地将资源的实际加载与需要资源的应用程序分离, URL只是一个字符串,资源加载也很容易配置。
This nicely decouples the actual loading of the resource from the application that needs the resource, and since a URL is just a String, resource loading is also very easily configurable.
是否有使用当前类加载器加载资源的协议?
这与Jar协议类似,不同之处在于我不需要知道资源来自哪个jar文件或类文件夹。
Is there a protocol to load resources using the current classloader? This is similar to the Jar protocol, except that I do not need to know which jar file or class folder the resource is coming from.
我可以做使用 <$当然,c $ c> Class.getResourceAsStream(a.xml) ,但这需要我使用不同的API,因此需要更改现有代码。我希望能够在所有我可以通过更新属性文件指定资源URL的地方使用它。
I can do that using Class.getResourceAsStream("a.xml")
, of course, but that would require me to use a different API, and hence changes to existing code. I want to be able to use this in all places where I can specify a URL for the resource already, by just updating a property file.
推荐答案
简介和基本实现
首先,您至少需要一个URLStreamHandler。这实际上将打开与给定URL的连接。请注意,这简称为 Handler
;这允许您指定 java -Djava.protocol.handler.pkgs = org.my.protocols
并使用简单包名称自动获取它支持的协议(在本例中为classpath)。
Intro and basic Implementation
First up, you're going to need at least a URLStreamHandler. This will actually open the connection to a given URL. Notice that this is simply called Handler
; this allows you to specify java -Djava.protocol.handler.pkgs=org.my.protocols
and it will automatically be picked up, using the "simple" package name as the supported protocol (in this case "classpath").
new URL("classpath:org/my/package/resource.extension").openConnection();
代码
Code
package org.my.protocols.classpath;
import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLStreamHandler;
/** A {@link URLStreamHandler} that handles resources on the classpath. */
public class Handler extends URLStreamHandler {
/** The classloader to find resources from. */
private final ClassLoader classLoader;
public Handler() {
this.classLoader = getClass().getClassLoader();
}
public Handler(ClassLoader classLoader) {
this.classLoader = classLoader;
}
@Override
protected URLConnection openConnection(URL u) throws IOException {
final URL resourceUrl = classLoader.getResource(u.getPath());
return resourceUrl.openConnection();
}
}
启动问题
如果你就像我一样,你不想依赖于在发布中设置的属性来让你到达某个地方(在我的情况下,我喜欢像Java WebStart一样保持我的选择 - 这就是为什么我需要所有这些)。Launch issues
If you're anything like me, you don't want to rely on a property being set in the launch to get you somewhere (in my case, I like to keep my options open like Java WebStart - which is why I need all this).如果您控制代码,则可以执行
If you control the code, you can do
new URL(null, "classpath:some/package/resource.extension", new org.my.protocols.classpath.Handler(ClassLoader.getSystemClassLoader()))
这将使用你的处理程序打开连接。
and this will use your handler to open the connection.
但是,这不太令人满意,因为你不需要一个URL到这样做 - 你想这样做,因为你不能(或不想)控制的某些lib需要url ...
But again, this is less than satisfactory, as you don't need a URL to do this - you want to do this because some lib you can't (or don't want to) control wants urls...
最终选项是注册 URLSt reamHandlerFactory
将处理jvm中的所有网址:
The ultimate option is to register a URLStreamHandlerFactory
that will handle all urls across the jvm:
package my.org.url;
import java.net.URLStreamHandler;
import java.net.URLStreamHandlerFactory;
import java.util.HashMap;
import java.util.Map;
class ConfigurableStreamHandlerFactory implements URLStreamHandlerFactory {
private final Map<String, URLStreamHandler> protocolHandlers;
public ConfigurableStreamHandlerFactory(String protocol, URLStreamHandler urlHandler) {
protocolHandlers = new HashMap<String, URLStreamHandler>();
addHandler(protocol, urlHandler);
}
public void addHandler(String protocol, URLStreamHandler urlHandler) {
protocolHandlers.put(protocol, urlHandler);
}
public URLStreamHandler createURLStreamHandler(String protocol) {
return protocolHandlers.get(protocol);
}
}
要注册处理程序,请致电已配置的工厂的URL.setURLStreamHandlerFactory()
。然后执行新URL(classpath:org / my / package / resource.extension)
,就像第一个例子一样,然后离开。
To register the handler, call URL.setURLStreamHandlerFactory()
with your configured factory. Then do new URL("classpath:org/my/package/resource.extension")
like the first example and away you go.
请注意,每个JVM只能调用一次此方法,并注意Tomcat将使用此方法注册JNDI处理程序(AFAIK)。试试码头(我会);在最坏的情况下,您可以先使用该方法然后它必须解决您的问题!
Note that this method may only be called once per JVM, and note well that Tomcat will use this method to register a JNDI handler (AFAIK). Try Jetty (I will be); at worst, you can use the method first and then it has to work around you!
我将此版本发布到公共领域,并询问您是否希望修改它在某处启动OSS项目并在此处评论详细信息。更好的实现是使用 URLStreamHandlerFactory
使用 ThreadLocal
来存储 URLStreamHandler 每个
Thread.currentThread()。getContextClassLoader()
的code> s。我甚至会给你我的修改和测试课程。
I release this to the public domain, and ask that if you wish to modify that you start a OSS project somewhere and comment here with the details. A better implementation would be to have a URLStreamHandlerFactory
that uses ThreadLocal
s to store URLStreamHandler
s for each Thread.currentThread().getContextClassLoader()
. I'll even give you my modifications and test classes.
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