java - How to get parameter types using reflection?

Question

I want to use functions having different numbers of parameters. The problem is that I don't know the number of parameters of each function, and also I don't know names of function as they are stored in an array. I only knows the class name, but don't want to use getDeclaredMethods as it will increase search time. Is there a way to get the parameter types for each function?

Answer 1

What I usually do when I have to look up methods is to generate a cache key from the query I am doing and save the search result with this cache key in a map.

Example:

I know the method parameters are Boolean.TRUE, Arrays.asList("foo","bar","baz") and BigInteger.valueOf(77777l)

My class contains a method with the signature

public foo(boolean, Collection, Number)

There's no way I can directly map the parameters to the parameter types because I just don't know which of the super classes or interfaces is the parameter type as you can see from the following table:

Expected Type          |  What I have
-----------------------------------------------------
 boolean               |  java.lang.Boolean
 java.util.Collection  |  java.util.Arrays$ArrayList
 java.lang.Number      |  java.math.BigInteger

Each of these pairs is compatible, but there's no way to find the compatible method without defining a comparison method, something like this:

// determine whether a method's parameter types are compatible
// with my arg array
public static boolean isCompatible(final Method method,
    final Object[] params) throws Exception{
    final Class<?>[] parameterTypes = method.getParameterTypes();
    if(params.length != parameterTypes.length){
        return false;
    }
    for(int i = 0; i < params.length; i++){
        final Object object = params[i];
        final Class<?> paramType = parameterTypes[i];
        if(!isCompatible(object, paramType)){
            return false;
        }
    }
    return true;
}

// determine whether a single object is compatible with
// a single parameter type
// careful: the object may be null
private static boolean isCompatible(final Object object,
    final Class<?> paramType) throws Exception{
    if(object == null){
        // primitive parameters are the only parameters
        // that can't handle a null object
        return !paramType.isPrimitive();
    }
    // handles same type, super types and implemented interfaces
    if(paramType.isInstance(object)){
        return true;
    }
    // special case: the arg may be the Object wrapper for the
    // primitive parameter type
    if(paramType.isPrimitive()){
        return isWrapperTypeOf(object.getClass(), paramType);
    }
    return false;

}

/*
  awful hack, can be made much more elegant using Guava:

  return Primitives.unwrap(candidate).equals(primitiveType);

*/
private static boolean isWrapperTypeOf(final Class<?> candidate,
    final Class<?> primitiveType) throws Exception{
    try{
        return !candidate.isPrimitive()
            && candidate
                .getDeclaredField("TYPE")
                .get(null)
                .equals(primitiveType);
    } catch(final NoSuchFieldException e){
        return false;
    } catch(final Exception e){
        throw e;
    }
}

So what I'd do is have a method cache:

private static final Map<String, Set<Method>> methodCache;

and add a lookup method like this:

public static Set<Method> getMatchingMethods(final Class<?> clazz,
    final Object[] args) throws Exception{
    final String cacheKey = toCacheKey(clazz, args);
    Set<Method> methods = methodCache.get(cacheKey);
    if(methods == null){
        final Set<Method> tmpMethods = new HashSet<Method>();
        for(final Method candidate : clazz.getDeclaredMethods()){
            if(isCompatible(candidate, args)){
                tmpMethods.add(candidate);
            }
        }
        methods = Collections.unmodifiableSet(tmpMethods);
        methodCache.put(cacheKey, methods);
    }
    return methods;
}

private static String toCacheKey(final Class<?> clazz, final Object[] args){
    final StringBuilder sb = new StringBuilder(clazz.getName());
    for(final Object obj : args){
        sb.append('-').append(
            obj == null ? "null" : obj.getClass().getName());
    }
    return sb.toString();
}

That way, subsequent lookups will take much less time than the first one (for parameters of the same type).

Of course since Class.getDeclaredMethods() uses a cache internally, the question is whether my cache improves performance at all. It's basically a question of what's faster:

1. generating a cache key and querying a HashMap or
2. iterating over all methods and querying for parameter compatibility

My guess: for large classes (many methods), the first method will win, otherwise the second will