There's something very unsatisfactory about this code:
/*
Given a command string in which the first 8 characters are the command name
padded on the right with whitespace, construct the appropriate kind of
Command object.
*/
public class CommandFactory {
public Command getCommand(String cmd) {
cmdName = cmd.subString(0,8).trim();
if(cmdName.equals("START")) {
return new StartCommand(cmd);
}
if(cmdName.equals("END")) {
return new EndCommand(cmd);
}
// ... more commands in more if blocks here
// else it's a bad command.
return new InvalidCommand(cmd);
}
}
I'm unrepentant about the multiple exit points - the structure is clear. But I'm not happy about the series of near-identical if statements. I've considered making a Map of Strings to Commands:
commandMap = new HashMap();
commandMap.put("START",StartCommand.class);
// ... etc.
... then using Reflection to make instances of the appropriate class looked up from the Map. However while conceptually elegant, this involves a fair amount of Reflection code that whoever inherits this code might not appreciate - although that cost might be offset by the benefits. All the lines hardcoding values into the commandMap smell almost as bad as the if block.
Even better would be if the factory's constructor could scan the classpath for subclasses of Command, query them for String representations, and automatically add them them to its repertoire.
So - how should I go about refactoring this?
I guess some of the frameworks out there give me this kind of thing for free. Let's assume I'm not in a position to migrate this stuff into such a framework.
How about the following code:
public enum CommandFactory {
START {
#Override
Command create(String cmd) {
return new StartCommand(cmd);
}
},
END {
#Override
Command create(String cmd) {
return new EndCommand(cmd);
}
};
abstract Command create(String cmd);
public static Command getCommand(String cmd) {
String cmdName = cmd.substring(0, 8).trim();
CommandFactory factory;
try {
factory = valueOf(cmdName);
}
catch (IllegalArgumentException e) {
return new InvalidCommand(cmd);
}
return factory.create(cmd);
}
}
The valueOf(String) of the enum is used to find the correct factory method. If the factory doesn't exist it will throw an IllegalArgumentException. We can use this as a signal to create the InvalidCommand object.
An extra benefit is that if you can make the method create(String cmd) public if you would also make this way of constructing a Command object compile time checked available to the rest of your code. You could then use CommandFactory.START.create(String cmd) to create a Command object.
The last benefit is that you can easily create a list of all available command in your Javadoc documentation.
Your map of strings to commands I think is good. You could even factor out the string command name to the constructor (i.e. shouldn't StartCommand know that its command is "START"?) If you could do this, instantiation of your command objects is much simpler:
Class c = commandMap.get(cmdName);
if (c != null)
return c.newInstance();
else
throw new IllegalArgumentException(cmdName + " is not as valid command");
Another option is to create an enum of all your commands with links to the classes (assume all your command objects implement CommandInterface):
public enum Command
{
START(StartCommand.class),
END(EndCommand.class);
private Class<? extends CommandInterface> mappedClass;
private Command(Class<? extends CommandInterface> c) { mappedClass = c; }
public CommandInterface getInstance()
{
return mappedClass.newInstance();
}
}
since the toString of an enum is its name, you can use EnumSet to locate the right object and get the class from within.
With the exception of the
cmd.subString(0,8).trim();
part, this doesn't look too bad to me. You could go with the Map and use reflection, but, depending on how often you add/change commands, this might not buy you much.
You should probably document why you only want the first 8 characters, or maybe change the protocol so it's easier to figure out which part of that string is the command (e.g. put a marker like ':' or ';' after the command key-word).
Its not directly an answer to your question, but why don't you throw an InvalidCommandException (or something similar), rather then returning an object of type InvalidCommand?
Unless there is a reason they can't be I always try to make my command implementations stateless. If that's the case you can add a method boolean identifier(String id) method to your command interface which would tell whether this instance could be used for the given string identifier. Then your factory could look something like this (note: I did not compile or test this):
public class CommandFactory {
private static List<Command> commands = new ArrayList<Command>();
public static void registerCommand(Command cmd) {
commands.add(cmd);
}
public Command getCommand(String cmd) {
for(Command instance : commands) {
if(instance.identifier(cmd)) {
return cmd;
}
}
throw new CommandNotRegisteredException(cmd);
}
}
I like your idea, but if you want to avoid reflection you could add instead instances to the HashMap:
commandMap = new HashMap();
commandMap.put("START",new StartCommand());
Whenever you need a command, you just clone it:
command = ((Command) commandMap.get(cmdName)).clone();
And afterwards, you set the command string:
command.setCommandString(cmdName);
But using clone() doesn't sound as elegant as using reflection :(
Taking a Convetion vs Configuration approach and using reflection to scan for available Command objects and loading them into your map would be the way to go. You then have the ability to expose new Commands without a recompile of the factory.
Another approach to dynamically finding the class to load, would be to omit the explicit map, and just try to build the class name from the command string. A title case and concatenate algorithm could turn "START" -> "com.mypackage.commands.StartCommand", and just use reflection to try to instantiate it. Fail somehow (InvalidCommand instance or an Exception of your own) if you can't find the class.
Then you add commands just by adding one object and start using it.
One option would be for each command type to have its own factory. This gives you two advantages:
1) Your generic factory wouldn't call new. So each command type could in future return an object of a different class according to the arguments following the space padding in the string.
2) In your HashMap scheme, you could avoid reflection by, for each command class, mapping to an object implementing a SpecialisedCommandFactory interface, instead of mapping to the class itself. This object in practice would probably be a singleton, but need not be specified as such. Your generic getCommand then calls the specialised getCommand.
That said, factory proliferation can get out of hand, and the code you have is the simplest thing that does the job. Personally I'd probably leave it as it is: you can compare command lists in source and spec without non-local considerations like what might have previously called CommandFactory.registerCommand, or what classes have been discovered through reflection. It's not confusing. It's very unlikely to be slow for less than a thousand commands. The only problem is that you can't add new command types without modifying the factory. But the modification you'd make is simple and repetitive, and if you forget to make it you get an obvious error for command lines containing the new type, so it's not onerous.
Having this repetitive object creation code all hidden in the factory is not so bad. If it has to be done somewhere, at least it's all here, so I'd not worry about it too much.
If you really want to do something about it, maybe go for the Map, but configure it from a properties file, and build the map from that props file.
Without going the classpath discovery route (about which I don't know), you'll always be modifying 2 places: writing a class, and then adding a mapping somewhere (factory, map init, or properties file).
Thinking about this, You could create little instantiation classes, like:
class CreateStartCommands implements CommandCreator {
public bool is_fitting_commandstring(String identifier) {
return identifier == "START"
}
public Startcommand create_instance(cmd) {
return StartCommand(cmd);
}
}
Of course, this adds a whole bunch if tiny classes that can't do much more than say "yes, thats start, give me that" or "nope, don't like that", however, you can now rework the factory to contain a list of those CommandCreators and just ask each of it: "you like this command?" and return the result of create_instance of the first accepting CommandCreator. Of course it now looks kind of akward to extract the first 8 characters outside of the CommandCreator, so I would rework that so you pass the entire command string into the CommandCreator.
I think I applied some "Replace switch with polymorphism"-Refactoring here, in case anyone wonders about that.
I'd go for the map and creation via reflection. If scanning the class path is too slow, you can always add a custom annotation to the class, have an annotation processor running at compile time and store all class names in the jar metadata.
Then, the only mistake you can do is forgetting the annotation.
I did something like this a while ago, using maven and APT.
The way I do it is to not have a generic Factory method.
I like to use Domain Objects as my command objects. Since I use Spring MVC this is a great approach since the DataBinder.setAllowedFields method allows me a great deal of flexibility to use a single domain object for several different forms.
To get a command object, I have a static factory method on the Domain object class. For example, in the member class I'd have methods like -
public static Member getCommandObjectForRegistration();
public static Member getCommandObjectForChangePassword();
And so on.
I'm not sure that this is a great approach, I never saw it suggested anywhere and kind of just came up with it on my own b/c I like the idea of keeping things like this in one place. If anybody sees any reason to object please let me know in the comments...
I would suggest avoiding reflection if at all possible. It is somewhat evil.
You can make your code more concise by using the ternary operator:
return
cmdName.equals("START") ? new StartCommand (cmd) :
cmdName.equals("END" ) ? new EndCommand (cmd) :
new InvalidCommand(cmd);
You could introduce an enum. Making each enum constant a factory is verbose and also has some runtime memory cost. But you can eaily lookup an enum and then use that with == or switch.
import xx.example.Command.*;
Command command = Command.valueOf(commandStr);
return
command == START ? new StartCommand (commandLine) :
command == END ? new EndCommand (commandLine) :
new InvalidCommand(commandLine);
Go with your gut, and reflect. However, in this solution, your Command interface is now assumed to have the setCommandString(String s) method accessible, so that newInstance is easily useable. Also, commandMap is any map with String keys (cmd) to Command class instances that they correspond to.
public class CommandFactory {
public Command getCommand(String cmd) {
if(cmd == null) {
return new InvalidCommand(cmd);
}
Class commandClass = (Class) commandMap.get(cmd);
if(commandClass == null) {
return new InvalidCommand(cmd);
}
try {
Command newCommand = (Command) commandClass.newInstance();
newCommand.setCommandString(cmd);
return newCommand;
}
catch(Exception e) {
return new InvalidCommand(cmd);
}
}
Hmm, browsing, and only just came across this. Can I still comment?
IMHO there's nothing wrong with the original if/else block code. This is simple, and simplicity must always be our first call in design (http://c2.com/cgi/wiki?DoTheSimplestThingThatCouldPossiblyWork)
This seems esp true as all the solutions offered are much less self documenting than the original code...I mean shouldn't we write our code for reading rather than translation...
At the very least, your command should have a getCommandString() -- where StartCommand overrides to return "START". Then you can just register or discover the classes.
+1 on the reflection suggestion, it will give you a more sane structure in your class.
Actually you could do the following (if you haven't thought about it already)
create methods corresponding to the String you'd be expecting as an argument to your getCommand() factory method, then all you have to do is reflect and invoke() these methods and return the correct object.
Related
I'm very new to programming language. My question might not even make sense. My environment is using java and trying to implement both ios and android apps in the same automation testing framework.
So, the idea is that any test script should be able to run on both the apps. Ex: one signin test script should be run for both ios and android.
I've decided to use interface and class implementation approach. The problem I'm facing is with test data. My company doesn't want to use excel. They want to use json for test data.
Here's my problem, look at the following line of code:
ValidBuy goodBuy = JsonFileReader.loadDaTa(TestBase.DATA_PATH, "good-buy.json", ValidBuy.class);
As you can see I have a class "ValidBuy" that has all the getters for a particular json file. I have another class "JsonFileReader" which takes the json filePath, fileName, and a class as an input and returns the data for that class name that I passed in. For this example I've passed ValidBuy.class
So, when I run a positive test, I'm passing "goodBuy" variable which is of type "ValidBuy". The problem starts here.
The test case is now specified with the data from goodBuy because it's type is "ValidBuy" and I'm passing goodBuy as a parameter.
Look at one of my extracted methods:
private void enterBuyInfo(ValidBuy goodBuy) {
itemPage = nativeApp.getItemPage(goodBuy);
itemPage.setItemName(goodBuy.getItemName());
itemPage.setItemSize(goodBuy.getItemSize());
itemPage.setItemDigitSSN(goodBuy.getSsn());
itemPage.clickContinue();
}
You can see those getters I'm using are coming from ValidBuy class.
If I run this test with the data for a badBuy:
InvalidBuy badBuy = JsonFileReader.loadDaTa(TestBase.DATA_PATH, "bad-buy.json", InvalidBuy.class);
It fails because now I have to change "ValidBuy" class with "InvalidBuy" class. Since, changing the parameter in the extracted method in every run is not possible, how can I make it more generic?
I want something like this:
TestData data = JsonFileReader.loadDaTa(RESOURCES_PATH, "good-client.json", InvalidBuy.class);
Here, TestData is generic. It could either be a class or interface (I don't know if that's possible) and the return type will be specified by whichever class I pass into the loadData() method. In this case InvalidBuy.class
The extracted method should look like this:
private void enterBuyInfo(TestData data) {
itemPage = nativeApp.getItemPage(data);
itemPage.setItemName(data.getItemName());
itemPage.setItemSize(data.getItemSize());
itemPage.setItemDigitSSN(data.getSsn());
itemPage.clickContinue();
}
If I can do this, I can use those extracted methods to create more tests.
I know I wrote a lot. I've only tried to make it as clear as possible. If it doesn't make any sense, just disregard it.
Any suggestions, ideas, code samples will be highly appreciated.
Firstly let me see if I understand your question. I think you are saying that loadData may return a value of type ValidBuy or InvalidBuy and you want to pass into it the class that you want returned. You then want to know how to use an interface that might represent either of these classes in your test methods so you can test various return values (both valid and invalid). You use the term "generic" in your question but I'm guessing you don't mean to use it in the specific way it's used in Java.
If I've understood your question correctly, then here's an answer:
Passing the class you wish to have returned into a method is an unusual usage and almost certainly not ideal. Better OOD would be to extract the common methods for all objects returned from loadData into an interface.
So:
interface Buy {
String getItemName();
boolean isValid();
}
class ValidBuy implements Buy {
#Override
public boolean isValid() {
return true;
}
...
}
class InvalidBuy implements Buy {
#Override
public boolean isValid() {
return false;
}
...
}
class JsonFileReader {
Buy loadData(Path path) {
...
}
}
Then your tests can look like:
#Test
void testValidBuy() {
assertTrue(reader.loadData(validPath).isvalid());
}
#Test
void testInvalidBuy() {
assertFalse(reader.loadData(invalidPath).isValid());
}
I realise I've simplified it a bit but hopefully you get the idea.
It sounds a stupid question. However, in the API that I am using, version 1.7.2 has got the method Bukkit.getServer().getOnlinePlayers() returning a Player[], and version 1.7.10 has got Bukkit.getServer().getOnlinePlayers() returning a Collection<Player>. I need to make my plugin compatible with both.
I have got the API's for both, but aside from creating separate plugins, I currently have got no idea how to do this.
I currently just convert the collection into an array anyway. So, is there any way to (i can already get the version) if the version is less than 1.7.9, not use the .toArray() but since it already returns an array?
You could do it with reflection. Something like this:
List<Player> getPlayers() {
try {
Method method = getMethod(Server.class, "getOnlinePlayers");
Object result = method.invoke(Bukkit.getServer());
if(result instanceof Player[])
return Arrays.asList((Player[])result);
else
return (List<Player>)result;
} catch(ReflectiveOperationException e) {
// something went wrong! If you have a better way to handle problems, do that instead
throw new RuntimeException(e);
}
}
There are actually two ways to do this. #immibis's answer gives the first way. The second way involves creating a "version adapter" API with multiple plugin implementations for different versions of (in this case) Bukkit; e.g.
public interface BukkitVersionAdapter {
Collection<Player> getOnlinePlayers();
...
}
public class BukkitVersionAdapterV1dot7 implements BukkitVersionAdapter {
public Collection<Player> getOnlinePlayers() {
return Arrays.asList(Bukkit.getServer().getOnlinePlayers());
}
...
}
public class BukkitVersionAdapterV1dot8 implements BukkitVersionAdapter {
public Collection<Player> getOnlinePlayers() {
return Bukkit.getServer().getOnlinePlayers();
}
...
}
The version-specific adapter classes then need to be compiled against the Bukkit API jars for the respective Bukkit versions.
Then when you start your main application (or Bukkit plugin I guess), you do something like this:
String version = // get the Bukkit version
String className = // map the version to a class name; e.g.
// "pkg.BukkitVersionAdapterV1dot7" or
// "pkg.BukkitVersionAdapterV1dot8"
Class clazz = Class.forName(className);
BukkitVersionAdapter adapter =
(BukkitVersionAdapter) clazz.newInstance();
// Then ...
Collection<Player> players = adapter.getOnlinePlayers();
This is all rather cumbersome, but it has two benefits compared with the approach of using reflection to make the calls:
The logic that is specific to different Bukkit versions is now isolated to one part of the codebase ... rather than being scattered (potentially) all over the place.
The overheads of reflection are only incurred at startup. After that, all of the method calls to Bukkit made via the adapter API are regular Java method calls.
Depending on the context, these may make the adapter approach more appropriate.
If the class org.bukkit.Server just changed the return type of the method getOnlinePlayers() rather than making it deprecated and introducing a new method (yack!!!!! who dares to change the already published interface in such a way?), you have no decent way how to call the method in one single code. You simply cannot write something like
Server server = Bukkit.getServer();
// One of the branches will not be compilable
if (version <= xxxx) {
Player[] players = server.getOnlinePlayers();
}
else {
Collection<Player> players = server.getOnlinePlayers();
}
I'm afraid the only way is to use reflection like in the meantime others recommended.
You are doing well. Unfortunately there is no way to create strongly typed method that can deal with both collection and array.
(You can create method that accepts Object and then examine it using instanceof, cast and deal with both collection and arrays but this solution is ugly.)
i was wondering if it's possible to initialize a constant in an interface from a property file using java or using spring messageSource, or such thing is not possible
please advise, thanks.
You can:
public interface Foo {
String a = Properties.getProperty("foo"); // public static final by default
}
However, that means that Properties.getProperty(..) has to be a static method which relies on an already initialized message source (statically again). Depending on the project and the frameworks you use this might not be the best option.
You could initialise a bean via a configuration which includes a final member. Since it's final you can assign to it during construction/initialisation and it then is immutable.
To configure from a property file using Spring, check out the PropertyPlaceholderConfigurer. That will allow you to initialise Spring beans using one or more property files from your classpath, filesystem, remote services etc.
Yes, that's possible:
public static final CONSTANT = System.getProperty("myProperty");
Although it's possible using some static helper method (as was already suggested), I would strongly recommend you not to do so for 2 reasons:
That looks like a pretty bad design. If you need a dynamic value - make it a method in the interface. Or use a static helper directly - you will need one anyway to make it work.
Constants might be inlined at compile time. That shouldn't happen in this particular case - compiler should go with inlining only if it can prove that value won't change between executions, basically if you initialize it with a literal, But there is a tiny chance that it would. Just think how bad will it be - no matter in which environment the progran is running, it picks up some useless value set during compilation, instead of what is configured. (This is rather a theoretical problem, need to say).
by reading a property file like in the example below.
int property1;
String otherProperty;
public void loadProperties(File propFile) throws IOException {
BufferedReader reader = new BufferedReader(new FileReader(propFile));
String line;
while ((line = reader.readLine()) != null) {
if (line.startsWith("Property1=")) {
property1 = Integer.parseInt(line.substring(10));
}
if (line.startsWith("OtherProperty=")) {
otherProperty = line.substring(14);
}
}
}
How can I change what a method is doing in Java ?
I mean, I am trying to use annotations to make the following code
#Anno1(Argument = "Option1")
public class TestClass
{
#Anno2
public void test()
{
}
}
Into
public class TestClass
{
private static StaticReference z;
public void test()
{
z.invokeToAll();
}
}
This is a very simplified example of what I am trying to do. Anno1 will have many possible combinations, but this is not my problem so far. My problem is how to add code to method test()
I am looking for a more generic solution if possible. Eg. A way to add every kind of code in the method (not just a way to .invokeToAll())
So far I am using import javax.annotation.processing.*; and I have the following code, but I don't know how to go on from there
private void processMethodAnnotations(RoundEnvironment env)
{
for (Element e : env.getElementsAnnotatedWith(Anno2.class))
{
//If it is a valid annotation over a method
if (e.getKind() == ElementKind.METHOD)
{
//What to do here :S
}else
{
processingEnv.getMessager().printMessage(Diagnostic.Kind.WARNING,"Not a method!", e);
}
}
}
I have found something about Java Reflection but I have not found any source to help me with what I am doing.
Obviously I extends AbstractProcessor in my code
I have found this tutorial (http://www.zdnetasia.com/writing-and-processing-custom-annotations-part-3-39362483.htm) But this concerns creating a new class, not just changing a method. and the javax.lang.model.elements do not provide any way of editing that element (which in my case represents a Method).
I hope my question is clear and inline with the rules. If not please comment and I will clarify. Thanks.
Annotation processing is the wrong way to go for you, from Wikipedia:
When Java source code is compiled,
annotations can be processed by
compiler plug-ins called annotation
processors. Processors can produce
informational messages or create
additional Java source files or
resources, which in turn may be
compiled and processed, but annotation
processors cannot modify the annotated
code itself.
People suggested to you the right way - AOP. Specifically, you can use AspectJ. "Quick result" way is (if you use Eclipse):
Install AJDT (AspectJ Development Tools)
Create an AspectJ project and add there your classes and annotations
Create Aspect:
public aspect Processor {
private StaticReference z;
pointcut generic()
// intercept execution of method named test, annotated with #Anno1
// from any class type, annotated with #Anno2
: execution(#Anno2 * (#Anno1 *).test())
// method takes no arguments
&& args ();
// here you have written what you want the method to actually do
void around () : generic() {
z.invokeToAll();
}
}
now you can execute a test and you will see that it works ;) AJDT compiles code for you automatically, so do not need any manual work to do, hope that's what you called "magic" ;)
UPDATE:
if your code in the test() method depends on the Anno1 annotation value, then inside aspect you can get class annotation for which it is executed this way:
void around () : generic() {
Annotation[] classAnnotations = thisJoinPoint.getThis().getClass().getAnnotations();
String ArgumentValue = null;
for ( Annotation annotation : classAnnotations ) {
if ( annotation instanceof Anno1 ) {
ArgumentValue = ((Anno1) annotation).Argument();
break;
}
}
if ( ArgumentValue != null && ArgumentValue.equals("Option1")) {
z.invokeToAll();
}
}
where thisJoinPoint is a special reference variable.
UPDATE2:
if you want to add System.out.println( this ) in your aspect, you need to write there System.out.println( thisJoinPoint.getThis() ), just tested and it works. thisJoinPoint.getThis() returns you "this" but not exactly; in fact this is Object variable and if you want to get any propery you need either to cast or to use reflection. And thisJoinPoint.getThis() does not provide access to private properties.
Well, now seems that your question is answered, but if I missed anything, or you get additional question/problems with this way - feel free to ask ;)
It's perfectly possible to do what you ask, although there is a caveat: relying on private compiler APIs. Sounds scary, but it isn't really (compiler implementations tend to be stable).
There's a paper that explains the procedure: The Hacker's Guide to Javac.
Notably, this is used by Project Lombok to provide automatic getter/setter generation (amongst other things). The following article explains how it does it, basically re-iterating what is said the aforementioned paper.
Well, you might see if the following boilerplate code will be useful:
public void magic(Object bean, String[] args) throws Exception {
for (Method method : bean.getClass().getDeclaredMethods()) {
if (method.isAnnotationPresent(Anno2.class)) {
// Invoke the original method
method.invoke(bean, args);
// Invoke your 'z' method
StaticReference.invokeAll();
}
}
}
As an alternative your might employ aspect oriented programming, for instance you have the AspectJ project.
I'm not sure at all if it is even possible to change the source or byte code via annotations. From what your describing it looks as if aspect oriented programming could provide a solution to your problem.
Your annotations are pretty similiar to the pointcut concept (they mark a location where code needs to be inserted) and the inserted code is close the advice concept.
Another approach would be parsing the java source file into an abstract syntax tree, modify this AST and serialize to a java compiler input.
If your class extends a suitable interface, you could wrap it in a DynamicProxy, which delegates all calls to the original methods, except the call to test.
My question is based around invoking a server method by using the command pattern. However there are many possible commands. I would like to introduce a return type for the 'execute' method of each command instance. I know this is not the intended use of the command pattern, but I haven't any other choice. I need to tailor the command pattern to work for me.
For the method signature of the 'execute' method in each command, what could I possibly have as the return type? I'm guessing it would have to be a covariant return type. It's not an ideal solution but I haven't many other options. I'm developing a server for my android app and RMI isn't available in the Android SDK. I would appreciate any advice about the return type issue. I would need to take account of all of the return types that could be returned from all of the different commands. I'm not sure if there is a pattern out there for this issue of returning some sort of generic return type.
I have already looked at this thread:
command pattern returning status
but I need more inspiration.
I probably don't understand the real question here, but this sounds straightforward.
public interface Command<T> {
T execute();
}
And then teh commands could be:
public class FooCommand implements Command<Bar> {
public Bar execute() {
...
}
}
Or is there a catch somewhere?
It looks like what you really want is RMI, which Android doesn't support.
You can try to use a lightweight RMI.
To implement something yourself, start from biziclop's interface,
// suppose there are object input/output streams established
// on client side
FooCommand foo = new FooCommand(params..);
Bar bar = remoteExec( foo );
<T> T remoteExec(Command<T> cmd)
output.writeObject(cmd);
return (T)input.readObject();
// on server side
Command cmd = (Command)input.readObject();
Object result = cmd.execute();
output.writeObject(result);