Problems start to appear when you need to:
- Make this class thread safe.
- Or, make a defensive copy of it due to difficulties with the first point.
Classical way around concurrency problems would be to use an immutable class. But managing immutable classes may be unwieldy, especially in the circumstances I've just described. You would need a huge constructor or a special builder class.
So this is the problem that I tried to solve and I hope you don't hate me after you read how I did it.
The idea was very simple – to use hashmap to store properties, but I didn't know how to make it type safe as values would be of different type. Then I came up with a special kind of key that dictates its value type. Here's an excerpt from PropertyKey.java:
public class PropertyKey<T> {
private final Class<T> clazz;
private final String name;
public PropertyKey(Class<T> valueType, String name) {
this.clazz = valueType;
this.name = name;
}
public boolean checkType(Object value) {
if (null == value) {
return true;
}
return this.clazz.isAssignableFrom(value.getClass());
}
}
Constructing new key looks like that:
PropertyKey<String> ENTITY_NAME =
new PropertyKey<String>(String.class, "ENTITY_NAME");
A bit verbose and I'm sorry for that, but what I get in return is a key for a field ENTITY_NAME of type String. Class has a method checkType that returns true if its argument extends T. All this is used by a special implementation of immutable hashmap.
public class PropertyHolder {
private final ImmutableMap<PropertyKey<?>, ?> storage;
public <T> T get(PropertyKey<T> key) {
return (T) storage.get(key);
}
/**
* Adds key/value pair to the state and returns new
* {@link PropertyHolder} with this state.
*
* @param key {@link PropertyKey} instance.
* @param value Value of type specified in {@link PropertyKey}.
* @return New {@link PropertyHolder} with updated state.
*/
public <T> PropertyHolder put(PropertyKey<T> key, T value) {
Preconditions.checkNotNull(key, "PropertyKey cannot be null");
Preconditions.checkNotNull(value, "Value for key %s is null",
key);
Preconditions.checkArgument(key.checkType(value),
"Property \"%s\" was given "
+ "value of a wrong type \"%s\"", key, value);
// Creates ImmutableMap.Builder with new key/value pair.
return new PropertyHolder(filterOutKey(key)
.put(key, value).build());
}
}
You can download full source code and complete unit tests from here. In full implementation I require values to be Serializable only because it is needed in my project, feel free to get rid of it.
PropertyHolder is an immutable class with interface very similar to Map. Its methods put and remove return new copy of PropertyHolder with modifications applied to it.
The way I do it is not very optimal as every modification creates full copy of ImmutableMap, one way to solve it would be to use persistent collections. But if you don't put there more than 20 objects and don't update it too often current implementation should do just fine.
Now declaring keys is a little ugly but once done it is very easy to use them:
// Adding new k/v pair PropertyHolder holder = new PropertyHolder(); holder = holder.put(ENTITY_NAME, "Person"); // Reading value for a key in a typesafe way String name = holder.get(ENTITY_NAME);
That's it. Please let me know if you find this solution useful or if there's a way to improve it.
Feel free to use it as is or modify it to your needs. I'm publishing full sources and unit tests under Eclipse Public License .
