reloadclass.html
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<TITLE>Dynamically Reloading a Modified Class
(Java Developers Almanac Example)
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<META CONTENT="Patrick Chan" NAME="AUTHOR">
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<h3>
e69.
Dynamically Reloading a Modified Class</h3>
This example demonstrates how to reload a modified class without
restarting the application. The technique involves loading the
<font color="#0066ff"><i>reloadable</i></font> class with a separate class loader. Each time the class
needs to be reloaded, it is loaded using a new class loader and the
previous class loader (with the old class) is abandoned.
<P> It is important that the reloadable class not be on the
classpath. Otherwise, the class will be loaded by some parent of the
new class loader rather than by the new class loader itself. Once this
happens, the class cannot be reloaded.
<P> Since the class cannot be on the classpath, it is not possible
to use the class name directly in the code (otherwise a
<code>ClassNotFoundException</code> would be thrown during start up). To circumvent
this problem, the reloadable class must be made to implement an
interface and the interface name is used in the code.
This example places the reloadable class in a subdirectory
called <code>dir</code>, which is not on the classpath. Here is the reloadable
class:
<pre>
public class MyReloadableClassImpl implements MyReloadableClass {
public String myMethod() {
return <font color="#0066ff"><i>"a message"</i></font>;
}
}
</pre>
To compile this class, it is necessary to tell the compiler
the location of <code>MyReloadableClass</code>. Since, for this example,
it is located in the parent directory, the following command will compile
this class:
<pre>
> java -classpath .. MyReloadableClass
</pre>
Here's the code that reloads the reloadable class:
<pre>
// Get the directory (URL) of the reloadable class
URL[] urls = null;
try {
// Convert the file object to a URL
File dir = new File(System.getProperty("user.dir")
+File.separator+<font color="#0066ff"><i>"dir"</i></font>+File.separator);
URL url = dir.toURL(); // file:/c:/almanac1.4/examples/
urls = new URL[]{url};
} catch (MalformedURLException e) {
}
try {
// Create a new class loader with the directory
ClassLoader cl = new URLClassLoader(urls);
// Load in the class
Class cls = cl.loadClass("MyReloadableClassImpl");
// Create a new instance of the new class
myObj = (MyReloadableClass)cls.newInstance();
} catch (IllegalAccessException e) {
} catch (InstantiationException e) {
} catch (ClassNotFoundException e) {
}
</pre>
Here's some code that tests the reloadable class (a more realistic
routine would periodically check the timestamp on the file). After the
example is started, change the string returned by <code>myMethod()</code> and
recompile.
<pre>
try {
while (true) {
reloadMyReloadedClass();
System.out.println(myObj.myMethod());
Thread.sleep(5000);
}
} catch (Exception e) {
}
</pre>
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Related Examples
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e59. <a class="eglink" href="GetClass.html?l=rel">
Getting a Class Object
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e60. <a class="eglink" href="GetClassName.html?l=rel">
Getting the Name of a Class Object
</a>
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e61. <a class="eglink" href="IsClass.html?l=rel">
Determining If a Class Object Represents a Class or Interface
</a>
<br>
e62. <a class="eglink" href="SuperClass.html?l=rel">
Getting the Superclass of an Object
</a>
<br>
e63. <a class="eglink" href="SuperClass2.html?l=rel">
Getting the Superclass of a Class Object
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e64. <a class="eglink" href="GetImplements2.html?l=rel">
Listing the Interfaces That a Class Implements
</a>
<br>
e65. <a class="eglink" href="GetImplements.html?l=rel">
Listing the Interfaces That an Interface Extends
</a>
<br>
e66. <a class="eglink" href="GetPkg.html?l=rel">
Getting the Package of a Class
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e67. <a class="eglink" href="ClassOrigin.html?l=rel">
Determining from Where a Class Was Loaded
</a>
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e68. <a class="eglink" href="LoadClass.html?l=rel">
Loading a Class That Is Not on the Classpath
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See also:
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Arrays">
Arrays
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Assertions">
Assertions
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Commands">
Commands
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Numbers">
Numbers
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Objects">
Objects
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Strings">
Strings
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#System%20Properties">
System Properties
</a>
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Threads">
Threads
</a>
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