blocks|key|3357831|text|我不相信你可以-正确的事情(我相信)是用新的路径创建一个新的类加载器。或者，您可以编写自己的类加载器，它允许您动态地更改类路径(用于该加载器)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3357832|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I don't believe you can - the right thing to do (I believe) is create a new classloader with the new path. Alternatively, you could write your own classloader which allows you to change the classpath (for that loader) dynamically.

blocks|key|357920|text|您可能想研究一下如何使用java.net.URLClassLoader。它允许您以编程方式加载最初不在类路径中的类，尽管我不确定这是否是您所需要的。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|357921|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/j2se/1.4.2/docs/api/java/net/URLClassLoader.html^0|C|N|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

You may want to look into using <a href="http://java.sun.com/j2se/1.4.2/docs/api/java/net/URLClassLoader.html" rel="nofollow noreferrer">java.net.URLClassLoader</a>. It allows you to programmatically load classes that weren't originally in your classpath, though I'm not sure if that's exactly what you need.

blocks|key|3145229|text|Update+Q4+2017:+as+commented+by+vda8888，在Java9中，系统java.lang.ClassLoader不再是java.net.URLClassLoader。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3145230|参见"Java+9+Migration+Guide:+The+Seven+Most+Common+Challenges“|3145231|3145232|，我刚刚描述的类加载策略是在一种新类型中实现的，在Java9中，应用程序类加载器就是这种类型。|blockquote|3145233|3145234|这意味着它不再是URLClassLoader，因此偶尔出现的(URLClassLoader)+getClass().getClassLoader()或(URLClassLoader)+ClassLoader.getSystemClassLoader()序列将不再执行。|3145235|3145236|java.lang.ModuleLayer将是一种替代方法，用于影响模块路径(而不是类路径)。例如，参见"Java+9+modules+-+JPMS+basics“。|3145237|3145238|对于Java+8或更低版本：|3145239|一些一般性的评论：|3145240|您不能(以一种保证工作的可移植方式，见下文)更改系统类路径。相反，您需要定义新的ClassLoader。|3145241|ClassLoaders以一种分层的方式工作...因此，对类X进行静态引用的任何类都需要装入与X相同的ClassLoader中，或者装入一个子ClassLoader中。您不能使用任何自定义ClassLoader来使由系统ClassLoader链接正确加载的代码，如果它以前没有这样做的话。因此，除了定位的额外代码之外，还需要安排在自定义ClassLoader中运行主要应用程序代码。|3145242|(也就是说，cracked-all在评论中提到了extending+the++URLClassLoader的这个例子)|3145243|您可以考虑不编写自己的ClassLoader，而只使用URLClassLoader。使用不在父类加载器url中的url创建一个URLClassLoader。|3145244|URL[]+url={new+URL("file://foo")};
URLClassLoader+loader+=+new+URLClassLoader(url);|code-block|syntax|javascript|3145245|more+complete+solution将为：|3145246|ClassLoader+currentThreadClassLoader
+=+Thread.currentThread().getContextClassLoader();

//+Add+the+conf+dir+to+the+classpath
//+Chain+the+current+thread+classloader
URLClassLoader+urlClassLoader
+=+new+URLClassLoader(new+URL[]{new+File("mtFile").toURL()},
++++++++++++++++++++++currentThreadClassLoader);

//+Replace+the+thread+classloader+-+assumes
//+you+have+permissions+to+do+so
Thread.currentThread().setContextClassLoader(urlClassLoader);|3145247|如果您假设JVM系统类加载器是一个URLClassLoader+(并不是所有JVM都是这样)，那么您也可以使用反射来实际修改系统类路径...(但这是一个黑客；))：|3145248|public+void+addURL(URL+url)+throws+Exception+{
++URLClassLoader+classLoader
+++++++++=+(URLClassLoader)+ClassLoader.getSystemClassLoader();
++Class+clazz=+URLClassLoader.class;

++//+Use+reflection
++Method+method=+clazz.getDeclaredMethod("addURL",+new+Class[]+{+URL.class+});
++method.setAccessible(true);
++method.invoke(classLoader,+new+Object[]+{+url+});
}

addURL(new+File("conf").toURL());

//+This+should+work+now!
Thread.currentThread().getContextClassLoader().getResourceAsStream("context.xml");|3145249|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/252893/how-do-you-change-the-classpath-within-java/252967#comment82290481_252967|1|https://stackoverflow.com/users/1974520/vda8888|2|https://docs.oracle.com/javase/9/docs/api/java/lang/ClassLoader.html|3|https://docs.oracle.com/javase/9/docs/api/java/net/URLClassLoader.html|4|https://blog.codefx.org/java/java-9-migration-guide/|5|https://docs.oracle.com/javase/9/docs/api/java/lang/ModuleLayer.html|6|http://blog.joda.org/2017/04/java-9-modules-jpms-basics.html|7|https://stackoverflow.com/users/188524/cracked-all|8|http://snippets.dzone.com/posts/show/3574|9|http://robertmaldon.blogspot.com/2007/11/dynamically-add-to-eclipse-junit.html^0|1E|L|22|N|J|9|0|W|7|1|1E|L|2|22|N|3|0|3|1K|4|0|0|0|0|8|E|U|18|23|1F|0|0|0|L|5|1H|S|6|0|0|0|0|0|0|13|E|6|B|7|O|T|8|0|0|0|0|M|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|27|8|@$9|28|A|29|B|C]|$9|2A|A|2B|B|C]]|D|@$9|2C|A|2D|1|2E]|$9|2F|A|2G|1|2H]|$9|2I|A|2J|1|2K]|$9|2L|A|2M|1|2N]]|E|$]]|$1|F|3|G|5|6|7|2O|8|@]|D|@$9|2P|A|2Q|1|2R]]|E|$]]|$1|H|3|-4|5|6|7|2S|8|@]|D|@]|E|$]]|$1|I|3|J|5|K|7|2T|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|2U|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|2V|8|@$9|2W|A|2X|B|C]|$9|2Y|A|2Z|B|C]|$9|30|A|31|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|32|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|33|8|@]|D|@$9|34|A|35|1|36]|$9|37|A|38|1|39]]|E|$]]|$1|R|3|-4|5|6|7|3A|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|3B|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|3C|8|@]|D|@]|E|$]]|$1|W|3|X|5|6|7|3D|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|3E|8|@]|D|@]|E|$]]|$1|10|3|11|5|6|7|3F|8|@$9|3G|A|3H|B|C]]|D|@$9|3I|A|3J|1|3K]|$9|3L|A|3M|1|3N]]|E|$]]|$1|12|3|13|5|6|7|3O|8|@]|D|@]|E|$]]|$1|14|3|15|5|16|7|3P|8|@]|D|@]|E|$17|18]]|$1|19|3|1A|5|6|7|3Q|8|@]|D|@$9|3R|A|3S|1|3T]]|E|$]]|$1|1B|3|1C|5|16|7|3U|8|@]|D|@]|E|$17|18]]|$1|1D|3|1E|5|6|7|3V|8|@]|D|@]|E|$]]|$1|1F|3|1G|5|16|7|3W|8|@]|D|@]|E|$17|18]]|$1|1H|3|-4|5|6|7|3X|8|@]|D|@]|E|$]]]|1I|$1J|$5|1K|1L|1M|E|$1N|1O]]|1P|$5|1K|1L|1M|E|$1N|1Q]]|1R|$5|1K|1L|1M|E|$1N|1S]]|1T|$5|1K|1L|1M|E|$1N|1U]]|1V|$5|1K|1L|1M|E|$1N|1W]]|1X|$5|1K|1L|1M|E|$1N|1Y]]|1Z|$5|1K|1L|1M|E|$1N|20]]|21|$5|1K|1L|1M|E|$1N|22]]|23|$5|1K|1L|1M|E|$1N|24]]|25|$5|1K|1L|1M|E|$1N|26]]]]

Update Q4 2017: as <a href="https://stackoverflow.com/questions/252893/how-do-you-change-the-classpath-within-java/252967#comment82290481_252967">commented</a> below by <a href="https://stackoverflow.com/users/1974520/vda8888">vda8888</a>, in Java 9, the System <a href="https://docs.oracle.com/javase/9/docs/api/java/lang/ClassLoader.html" rel="nofollow noreferrer"><code>java.lang.ClassLoader</code></a> is no longer a <a href="https://docs.oracle.com/javase/9/docs/api/java/net/URLClassLoader.html" rel="nofollow noreferrer"><code>java.net.URLClassLoader</code></a>.

See "<a href="https://blog.codefx.org/java/java-9-migration-guide/" rel="nofollow noreferrer">Java 9 Migration Guide: The Seven Most Common Challenges</a>"

<blockquote>
 The class loading strategy that I just described is implemented in a new type and in Java 9 the application class loader is of that type. 
 That means it is not a <code>URLClassLoader</code> anymore, so the occasional <code>(URLClassLoader) getClass().getClassLoader()</code> or <code>(URLClassLoader) ClassLoader.getSystemClassLoader()</code> sequences will no longer execute.
</blockquote>

<a href="https://docs.oracle.com/javase/9/docs/api/java/lang/ModuleLayer.html" rel="nofollow noreferrer">java.lang.ModuleLayer</a> would be an alternative approach used in order to influence the modulepath (instead of the classpath). See for instance "<a href="http://blog.joda.org/2017/04/java-9-modules-jpms-basics.html" rel="nofollow noreferrer">Java 9 modules - JPMS basics</a>".

<hr>

For Java 8 or below:

Some general comments:

you cannot (in a portable way that's guaranteed to work, see below) change the system classpath. Instead, you need to define a new ClassLoader.

ClassLoaders work in a hierarchical manner... so any class that makes a static reference to class X needs to be loaded in the same ClassLoader as X, or in a child ClassLoader. You can NOT use any custom ClassLoader to make code loaded by the system ClassLoader link properly, if it wouldn't have done so before. So you need to arrange for your main application code to be run in the custom ClassLoader in addition to the extra code that you locate. 
(That being said, <a href="https://stackoverflow.com/users/188524/cracked-all">cracked-all</a> mentions in the comments this example of <a href="http://snippets.dzone.com/posts/show/3574" rel="nofollow noreferrer">extending the <code>URLClassLoader</code></a>)

And you might consider not writing your own ClassLoader, but just use URLClassLoader instead. Create a URLClassLoader with a url that are not in the parent classloaders url's.

<pre><code>URL[] url={new URL("file://foo")};
URLClassLoader loader = new URLClassLoader(url);
</code></pre>

A <a href="http://robertmaldon.blogspot.com/2007/11/dynamically-add-to-eclipse-junit.html" rel="nofollow noreferrer">more complete solution</a> would be:

<pre><code>ClassLoader currentThreadClassLoader
 = Thread.currentThread().getContextClassLoader();

// Add the conf dir to the classpath
// Chain the current thread classloader
URLClassLoader urlClassLoader
 = new URLClassLoader(new URL[]{new File("mtFile").toURL()},
 currentThreadClassLoader);

// Replace the thread classloader - assumes
// you have permissions to do so
Thread.currentThread().setContextClassLoader(urlClassLoader);
</code></pre>

If you assume the JVMs system classloader is a URLClassLoader (which may not be true for all JVMs), you can use reflection as well to actually modify the system classpath... (but that's a hack;)):

<pre><code>public void addURL(URL url) throws Exception {
 URLClassLoader classLoader
 = (URLClassLoader) ClassLoader.getSystemClassLoader();
 Class clazz= URLClassLoader.class;

 // Use reflection
 Method method= clazz.getDeclaredMethod("addURL", new Class[] { URL.class });
 method.setAccessible(true);
 method.invoke(classLoader, new Object[] { url });
}

addURL(new File("conf").toURL());

// This should work now!
Thread.currentThread().getContextClassLoader().getResourceAsStream("context.xml");
</code></pre>

blocks|key|3357976|text|从下面的两个链接可以看出，VonC给出的方法似乎是最好的，但请查看其中一些帖子和谷歌的"Java+Dynamic+Classpath“或"Java+Dynamic+Class+Loading”，并从中找到一些信息。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3357977|我会发表更深入的文章，但VonC已经基本上完成了这项工作。|3357978|来自Dynamic+loading+of+class+and+Jar+files。|offset|length|3357979|还要检查这个sun+forum+post。|3357980|entityMap|0|LINK|mutability|MUTABLE|url|http://twit88.com/blog/2007/10/04/java-dynamic-loading-of-class-and-jar-file/|1|http://forums.sun.com/thread.jspa?threadID=300557^0|0|0|2|12|0|0|6|E|1|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|V|8|@]|9|@$F|W|G|X|1|Y]]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@$F|10|G|11|1|12]]|A|$]]|$1|J|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q]]|R|$5|M|N|O|A|$P|S]]]]

It is possible as seen from the two links below, the method VonC gives seems to be the best but check out some of these posts and google for "Java Dynamic Classpath" or "Java Dynamic Class Loading" and find out some info from there.

I'd post in more depth but VonC has pretty much done the job.

From <a href="http://twit88.com/blog/2007/10/04/java-dynamic-loading-of-class-and-jar-file/" rel="nofollow noreferrer">Dynamic loading of class and Jar files</a>.

Also check this <a href="http://forums.sun.com/thread.jspa?threadID=300557" rel="nofollow noreferrer">sun forum post</a>.

blocks|key|3357982|text|不需要编写自己的类加载器！那是clojure.lang.DynamicClassLoader。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3357983|http://blog.japila.pl/2011/01/dynamically-redefining-classpath-in-clojure-repl/|3357984|entityMap|0|LINK|mutability|MUTABLE|url|http://code.google.com/p/clojure/source/browse/trunk/src/jvm/clojure/lang/DynamicClassLoader.java|1^0|F|V|0|0|0|27|1|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@$A|P|B|Q|1|R]]|C|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|F|3|-4|5|6|7|W|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|E]]]]

There's no need to write your own class loader! There's <a href="http://code.google.com/p/clojure/source/browse/trunk/src/jvm/clojure/lang/DynamicClassLoader.java" rel="nofollow">clojure.lang.DynamicClassLoader</a>.

<a href="http://blog.japila.pl/2011/01/dynamically-redefining-classpath-in-clojure-repl/" rel="nofollow">http://blog.japila.pl/2011/01/dynamically-redefining-classpath-in-clojure-repl/</a>

blocks|key|3145297|text|String+s="java++-classpath+abcd/+"%2Bpgmname%2B"+"%2Bfilename;+++
Process+pro2+=+Runtime.getRuntime().exec(s);+
BufferedReader+in+=+new+BufferedReader(new+InputStreamReader(pro2.getInputStream()));|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3145298|是在java程序中更改类路径的一个示例。|unstyled|3145299|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>String s="java -classpath abcd/ "+pgmname+" "+filename; 
Process pro2 = Runtime.getRuntime().exec(s); 
BufferedReader in = new BufferedReader(new InputStreamReader(pro2.getInputStream()));
</code></pre>

is an example of changin the classpath in java program

How do you change the CLASSPATH of a Java process from within the Java process?

<hr>

Before you ask me "Why would you want to do that?" I'll explain it shortly. 

<blockquote>
 When you have a Clojure REPL running it is common to need more jars in your CLASSPATH to load a <a href="http://clojure.org" rel="noreferrer">Clojure</a> source file, and I'd like to do it without having to restart Clojure itself (which is not really an option when using it on Slime on Emacs).
</blockquote>

That's the reason but I don't want this question tagged as some-weird-language some-weird-editor and be disregarded by the majority of Java developers that may have the answer.

How do you change the CLASSPATH within Java?

Java

如何在Java进程中更改Java进程的CLASSPATH？在你问我“你为什么要这么做？”我很快就会解释它。当您运行Clojure REPL时，通常需要在您的CLASSPATH中使用更多的jars来加载源文件，我希望不必重启Clojure本身(在Emacs上的Slime上使用它时，这并不是一个真正的选择)。这就是原因，但...

问如何在Java中更改CLASSPATH？
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