Adam Connor
Posts: 12
Nickname: adamconnor
Registered: Jan, 2004
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Re: instanceExists() ...
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Posted: Jan 22, 2004 5:54 PM
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I think this is a tricky question to answer. Assuming that finding out if the class has been loaded is good enough, the method you want is findLoadedClass(String name) in ClassLoader; it will return null if the class has not been loaded.
However, it is a protected final method, so you can't call it directly. The following program outlines one approach for accessing it:
public class TestClassIsLoaded {
public static void main(String[] args) throws Exception {
TestClassIsLoaded test = new TestClassIsLoaded();
System.out.println("BigDecimal loaded? " + test.isLoaded("java.math.BigDecimal") );
System.out.println("String loaded? " + test.isLoaded("java.lang.String"));
BigDecimal bd = new BigDecimal(0);
System.out.println("BigDecimal loaded? " + test.isLoaded("java.math.BigDecimal") );
}
public boolean isLoaded(String className)
throws IllegalAccessException, InvocationTargetException {
ClassLoader loader = TestClassIsLoaded.class.getClassLoader();
Class lc = loader.getClass();
while( lc != null ){
try {
Method m = lc.getDeclaredMethod( "findLoadedClass", new Class[]{String.class} );
// next line requires "suppressAccessChecks" permission if security is on
m.setAccessible(true);
Object result = m.invoke(loader, new Object[]{className});
return result != null;
}
catch (NoSuchMethodException e) {
lc = lc.getSuperclass();
continue;
}
}
throw new IllegalStateException("Never found \"findLoadedClass\".");
}
}
If you are running with the SecurityManager turned on, you will need sufficient permissions to perform these operations.
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