Entries tagged [design-patterns]
The State Pattern
TweetPosted on Thursday Aug 11, 2022 at 01:21PM in Technology
What is the State Pattern?
It’s one of the GoF Design Patterns which helps to organize code that handles multiple states. If you have a code base where there are many boolean fields or an enum field used to determine the state, similar if/switch branches and unclear state transitions, a giant class where there is all of the stuff in messy code causing a maintainability issue, applying the State Pattern might be worth considering.
The State Pattern consists of the following participants:
The Context class provides the clients of your code with the public API. All of the other participants are implementation details which the clients of your class don’t have to know about.
The Context class holds a reference to an object which implements the State interface. The Context class forwards some of the API calls to the state object. The State interface defines part of the API whose behavior has to be changed depending on the state.
The ConcreteState classes implement the State interface. Each implementation exists for each possible state of your application. It handles the API calls forwarded from the Context class appropriately according to the requirement of the state it corresponds to. A ConcreteState class can trigger a state transition.
Case study: the SMTP protocol
As an example application for the State Pattern, let’s think about an SMTP server. SMTP is a protocol used for email transport. Usually email client software like Mozilla Thunderbird supports this protocol. When such software sends an email, it connects to an SMTP server and talks to the server using this protocol to send an email.
This is a typical flow of an SMTP communication session (taken from Wikipedia):
S: 220 smtp.example.com ESMTP Postfix
C: HELO relay.example.com
S: 250 smtp.example.com, I am glad to meet you
C: MAIL FROM:<bob@example.com>
S: 250 Ok
C: RCPT TO:<alice@example.com>
S: 250 Ok
C: RCPT TO:<theboss@example.com>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: "Bob Example" <bob@example.com>
C: To: Alice Example <alice@example.com>
C: Cc: theboss@example.com
C: Date: Tue, 15 Jan 2008 16:02:43 -0500
C: Subject: Test message
C:
C: Hello Alice.
C: This is a test message with 5 header fields and 4 lines in the message body.
C: Your friend,
C: Bob
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye
{The server closes the connection}The lines beginning with "S:" are sent by the server and the ones beginning with "C: " are sent by the client.
Typically, after establishing a TCP connection, the server sends a short message which indicates it is ready to accept a command from the client. The client first sends an HELO message with its hostname, then begins an email transaction with specifying the email address of the sender of the email with a MAIL FROM command. After that, the client sends the email addresses of the recipients of the email with RCPT TO commands. Then finally, the client sends a DATA command, sends the content of the email and finishes it with a line which contains only a period. If everything goes fine, the server responds with an OK message which means the email is accepted by the server and the email will be delivered to the recipients specified.
What we can see from here is that in this protocol a client has to send necessary information to the server in a specific order. The server reacts differently for each state of the communication process. For example, the client must provide a sender identification first. Otherwise, any other command from the client doesn’t get processed and the server returns an error. It means that the server remembers the current communication state at any given moment.
So, what are those states? A good way to find that out is drawing a state diagram. It will look like the following:
There are 4 states in the communication process. The first one is the idle state where the client has to start a session with an HELO command. Then it proceeds to the initial state, where the client has to provide the email address of the sender of the email with a MAIL FROM command. Then we proceed to the transaction started state where the client provides the recipients of the email with an RCPT TO command. The client might stay in this state until it finishes sending all of the recipients and that is why there is this transition which goes back to the same state. After that, finally it proceeds to the data transfer state with a DATA command where the client sends the body of the email and finishes the transaction with a line which contains only a period. Then it gets back to the initial state and if the client wants to send another email, it can start over from there.
Implementing an SMTP server with the State Pattern
In order to implement such a communication process which consists of multiple stages or states, applying the State Pattern can be a good way to keep the implementation clean and organized. Otherwise, we might end up having a giant class where there are a lot of mutable states and if/switch conditionals which are highly likely to be a maintenance problem.
Let’s look at one possible design based on the State Pattern which can handle the SMTP protocol:
The SMTPSessionHandler class corresponds to the Context class in the previous class diagram. An instance of this class exists for each SMTP conversation session. There is a public method called handleCommand() which receives a command from the client and returns the response for the command. This method has to handle the commands from the client appropriately depending on the current state. In order to achieve that, we have those 4 concrete state classes that correspond to the states in the state diagram given earlier, and what this method does is basically just forwarding the method calls to the currentState object.
In this design, state transitions are done by each concrete state class. For that purpose, the SMTPSessionHandler class has the setCurrentState() method. And when the SMTPSessionHandler class forwards the method calls to the currentState object, it also passes its own reference as an additional parameter. It allows each concrete state class to call the setCurrentState() method.
Let’s look at the source code of each participant. The SMTPSessionHandler class just holds the currentState object and forwards any handleCommand() calls to the currentState object with a reference to this object. It also has the setCurrentState() method for the concrete state classes.
public class SMTPSessionHandler {
private State currentState = new IdleState();
public String handleCommand(String command) {
return currentState.handleCommand(command, this);
}
void setCurrentState(State newState) {
this.currentState = newState;
}
}The SMTPSessionHandler class calls the underlying current state object through this State interface, which has 4 concrete implementations.
interface State {
String handleCommand(String command, SMTPSessionHandler context);
}The IdleState class corresponds to the very first state when a connection is established with the client. The only command it accepts is the HELO command. When it receives the HELO command, it makes a state transition by calling the setCurrentState() method of the context object with a new instance of the InitialState class and returns a greeting to the client. Otherwise, it returns an error.
class IdleState implements State {
@Override
public String handleCommand(String command, SMTPSessionHandler context) {
if (command.startsWith("HELO")) {
context.setCurrentState(new InitialState());
return "250 smtp.example.com, I am glad to meet you";
} else {
return "500 5.5.1 Invalid command";
}
}
}When the state transition in the IdleState class happens, the InitialState class takes over. The only command it accepts is the MAIL FROM command. When it happens, it extracts the email address which the client has sent and makes another state transition with a new instance of the TransactionStartedState class. When it creates the instance, it passes the email address it has extracted in order to let the subsequent process use it. Otherwise it returns an error.
class InitialState implements State {
private static final Pattern PATTERN_FOR_EXTRACTING_EMAIL =
Pattern.compile("MAIL FROM:<([^>]+)>");
@Override
public String handleCommand(String command, SMTPSessionHandler context) {
Matcher matcher = PATTERN_FOR_EXTRACTING_EMAIL.matcher(command);
if (matcher.find()) {
String from = matcher.group(1);
context.setCurrentState(new TransactionStartedState(from));
return "250 Ok";
} else {
return "500 5.5.1 Invalid command";
}
}
}The TransactionStartedState class is where we receive the destinations of the email. After specifying at least one destination, we can proceed to the next state but if there is none, it returns an error. The client has to send the destinations with the RCPT TO command. When it receives the RCPT TO command, it extracts the email address and keeps it in the List object. After sending at least one destination, the client can proceed to the next state with the DATA command. At this point, we have the address of the sender and the destinations and those are passed as the parameters of the constructor of the DataTransferState class.
class TransactionStartedState implements State {
private static final Pattern PATTERN_FOR_EXTRACTING_EMAIL =
Pattern.compile("RCPT TO:<([^>]+)>");
private final String from;
private final List<String> destinations = new ArrayList<>();
TransactionStartedState(String from) {
this.from = from;
}
@Override
public String handleCommand(String command, SMTPSessionHandler context) {
if (command.equals("DATA")) {
if (destinations.isEmpty()) {
return "500 5.5.1 Invalid command";
} else {
context.setCurrentState(new DataTransferState(from, destinations));
return "354 End data with <CR><LF>.<CR><LF>";
}
}
Matcher matcher = PATTERN_FOR_EXTRACTING_EMAIL.matcher(command);
if (matcher.find()) {
String to = matcher.group(1);
destinations.add(to);
return "250 Ok";
} else {
return "500 5.5.1 Invalid command";
}
}
}The DataTransferState class corresponds to the final state of this communication process. What it does is just accumulating the body of the email in a StringBuilder object until it receives one single period which means the end of the body. After that, it will trigger the email delivery process which involves a DNS lookup, relaying the message to another SMTP server using the pieces of the data we received from the client during the process and making a state transition to the initial state. If the client wants to send another email, the client can start all over from there.
class DataTransferState implements State {
private final String from;
private final List<String> destinations;
private final StringBuilder body = new StringBuilder();
static DeliverySystem deliverySystem = (from, destinations, body) -> {
// looks up MX records, connects to external SMTP servers and relays the message
};
DataTransferState(String from, List<String> destinations) {
this.from = from;
this.destinations = destinations;
}
@Override
public String handleCommand(String command, SMTPSessionHandler context) {
if (command.equals(".")) {
deliverySystem.deliver(from, destinations, body.toString());
context.setCurrentState(new InitialState());
return "250 Ok: the email has been delivered";
} else {
body.append(command);
body.append('\n');
return null;
}
}
}Conclusion: benefits of the State Pattern
That’s all of the implementation. The responsibility of each concrete state class is clear and it helps to make each class concise and short. The state transitions are also clear because when they happen, they are done in a clear way, which is the setCurrentState() method in our example. There are no cryptic boolean flags that maintain states in an unclear way. It makes the code readable, easy to maintain and extend.
For example, when you need to add a new state, what you need to do is write a new class which corresponds to the new state and add state transitions to the relevant existing state classes that are likely to be much simpler than a giant single class maintaining all of the states. It will reduce the risk of degradation.
In my experience, there are not so many use cases where this pattern is a great match, but it greatly improves the maintainability if it matches the use case and is applied appropriately.
Tags: design-patterns
The Singleton Pattern in Java
TweetPosted on Tuesday Sep 22, 2020 at 12:47PM in Technology
In this entry, I’ll introduce some common ways to apply the Singleton Pattern in Java and a problem that can harm maintainability of your code base and a solution for the problem.
What is it?
The Singleton Pattern is a commonly used idiom to create and maintain objects called singletons. A singleton is the only instance of a class.
Common use cases are an object which keeps the user’s preferences in a desktop application, cache or a resource pool. One example would be a TCP connection pool where there are multiple active connections to a backend service like a database server or a web service. Typically there should be only one pool object which maintains all of the connections. Having multiple separate pool objects in one application doesn’t make much sense in most cases because it will make the use of the pool less effective.
As an example, let’s imagine that there is the following interface which allows us to fetch the weather data from some external web service:
interface WeatherFetcher {
String fetchWeather(String cityCode) throws IOException;
int fetchTemperature(String cityCode) throws IOException;
}There are a couple of methods to fetch the weather (atmospheric conditions) and the temperature of a city. Since it involves network communcation, there can be an IOException.
Now imagine that you are working on implementing a feature which uses this interface. You are told that the performance requirement for this feature is strict, therefore you have decided to maintain a pool of active TCP connections to the external web service in order to keep the latency low. Applying the Singleton Pattern here sounds like a good idea because it will make sure that you will have only one pool object in your application.
Class with a private constructor
Let’s see how we can implement this with the Singleton Pattern. One simple implementation would be something like this:
public class SimpleSingleton implements WeatherFetcher {
private static final SimpleSingleton INSTANCE = new SimpleSingleton();
public static WeatherFetcher getInstance() { return INSTANCE; }
private SimpleSingleton() { System.out.println("Populating the connection pool..."); }
@Override
public int fetchTemperature(String cityCode) {
// gets an idle connection from the pool, sends a request
return 71; // please assume it's from a response
}
@Override
public String fetchWeather(String cityCode) {
// gets an idle connection from the pool, sends a request
return "Sunny"; // please assume it's from a response
}
// client code
public static void main(String[] args) throws IOException {
WeatherFetcher singleton = SimpleSingleton.getInstance();
System.out.printf("Weather in New York: %s, %dF\n",
singleton.fetchWeather("NewYork"), singleton.fetchTemperature("NewYork"));
}
}The class has a static final field called INSTANCE, where the only instance of this class is kept. There is a simple getter for that.
This class has a constructor populating the pool. Let’s imagine that it opens a lot of connections to an external web service and makes those ready to use. The constructor is private, which prevents creation of another instance of the class. Without this, any other class can create an instance of this class, which means that there is no guarantee that there is only one instance of this class. It also prevents inheritance because there is no constructor a subclass can call.
In order to get the singleton, the client needs to call the static method getInstance() first. After that, the client can fetch the weather data via the methods defined in the WeatherFetcher interface.
Class with a static holder class for lazy initialization
The SimpleSingleton class works fine, but there is one common requirement you might encounter. Populating the connection pool can be expensive and there might be a situation where the feature we just implemented will be used by only a very small number of your users, and you want to make sure that the connection pool gets populated only when it’s really needed.
We cannot guarantee that with the SimpleSingleton class. Let’s add a System.out.println() call to the main method and see what is happening there:
public static void main(String[] args) throws IOException {
System.out.println("Doing some other stuff - I don't need the singleton yet"); // added
WeatherFetcher singleton = SimpleSingleton.getInstance();
System.out.printf("Weather in New York: %s, %dF\n",
singleton.fetchWeather("NewYork"), singleton.fetchTemperature("NewYork"));
}The main method yields the following output:
Populating the connection pool...
Doing some other stuff - I don't need the singleton yet
Weather in New York: Sunny, 71FAs you can see, the pool has got initialized even though it’s not needed yet; the constructor gets executed before the getInstance() method is called because the constructor invocation is written in the static initializer of the class. It can lead to a situation where unnecessary resource consumption is imposed on the user even though the user might not need the feature. We can work around this with having a simple private static holder class like the following:
public class LazySingleton implements WeatherFetcher {
private static class SingletonHolder {
static final LazySingleton INSTANCE = new LazySingleton();
}
public static WeatherFetcher getInstance() { return SingletonHolder.INSTANCE; }
private LazySingleton() { System.out.println("Populating the connection pool..."); }
...
// client code
public static void main(String[] args) throws IOException {
System.out.println("Doing some other stuff - I don't need the singleton yet");
WeatherFetcher singleton = LazySingleton.getInstance();
System.out.printf("Weather in New York: %s, %dF\n",
singleton.fetchWeather("NewYork"), singleton.fetchTemperature("NewYork"));
}
}The LazySingleton singleton instance is kept in the static field in the static inner class called SingletonHolder and the getInstance() method returns that. Executing the main method will show the following output, as expected:
Doing some other stuff - I don't need the singleton yet
Populating the connection pool...
Weather in New York: Sunny, 71FA problem that can harm maintainability: getInstance() everywhere
Let’s imagine that now you need to implement some code for generating a report about the weather of some cities with the singleton we just wrote. It might be implemented like this:
public class WeatherReporter {
String generateWeatherReport(List<String> cityCodeList) {
StringBuilder sb = new StringBuilder("=== Weather Report ===\n");
for (String cityCode : cityCodeList) {
try {
String weather = LazySingleton.getInstance().fetchWeather(cityCode);
int temperature = LazySingleton.getInstance().fetchTemperature(cityCode);
sb.append(String.format("%s: %s, %dF\n", cityCode, weather, temperature));
} catch (IOException e) {
sb.append(String.format("%s: Failed to fetch data\n", cityCode));
}
}
return sb.toString();
}
public static void main(String[] args) {
WeatherReporter reporter = new WeatherReporter();
System.out.println(reporter.generateWeatherReport(
Arrays.asList("NewYork", "Montreal", "Tokyo")));
}
}It receives a List of city code which we need to create the report for. Then it constructs the header, iterates the list of city code and fetches the weather and the temperature for each city and builds the String which represents the report. If there is an I/O problem, we just mention that it failed to fetch the data for the city.
Executing the main method should produce the following output:
Populating the connection pool...
=== Weather Report ===
NewYork: Sunny, 71F
Montreal: Cloudy, 68F
Tokyo: Rainy, 69FThat’s a fairly complicated piece of code. Therefore, having some unit tests for the WeatherReporter class would be nice, but unfortunately it’s almost impossible because the current implementation is hard-coded to directly call the static getInstance() method to get the WeatherFetcher object, which means it always sends requests to the real external web service.
We cannot make a reliable assertion if it depends on the output of the real external service that way. One option would be running a fake of the external service, but it would require a lot of effort. And it would be nice if we could write a test case for IOException handling, but reproducing IOException in a real environment is also too much work. And the connection pool population might take a lot of time. The LazySingleton class might be hard-coded to create thousands of TCP connections and in that case it might take a few tens of seconds to populate. Waiting for such a long time with every unit test execution doesn’t sound reasonable.
So, the problem here is that we have business logic tightly coupled to an external data source. Having that kind of a method call in the middle of business logic makes writing unit tests very difficult or almost impossible because in order to make that kind of thing work, in many cases it requires some sort of specific setup or configuration and oftentimes that is too much work.
One thing we can try to work around this problem is replacing the static getInstance() method call by a functional interface. In our case, we can do something like this:
public class ImprovedWeatherReporter {
private final Supplier<? extends WeatherFetcher> weatherFetcherSupplier;
public ImprovedWeatherReporter(Supplier<? extends WeatherFetcher> weatherFetcherSupplier) {
this.weatherFetcherSupplier = weatherFetcherSupplier;
}
String generateWeatherReport(List<String> cityCodeList) {
StringBuilder sb = new StringBuilder("=== Weather Report ===\n");
for (String cityCode : cityCodeList) {
try {
String weather = weatherFetcherSupplier.get().fetchWeather(cityCode);
int temperature = weatherFetcherSupplier.get().fetchTemperature(cityCode);
sb.append(String.format("%s: %s, %dF\n", cityCode, weather, temperature));
} catch (IOException e) {
sb.append(String.format("%s: Failed to fetch data\n", cityCode));
}
}
return sb.toString();
}
public static void main(String[] args) {
ImprovedWeatherReporter reporter = new ImprovedWeatherReporter(LazySingleton::getInstance);
System.out.println(reporter.generateWeatherReport(
Arrays.asList("NewYork", "Montreal", "Tokyo")));
}
}The ImprovedWeatherReporter class is not tightly coupled to the getInstance() method anymore. Instead, now it has a field for a Supplier object which returns a WeatherFetcher object. The client of this class can inject the Supplier via its constructor. In our case, we can inject LazySingleton.getInstance() for the production use.
That seems to be not much change, but it is a great improvement from the perspective of unit testing. Now we can inject any Supplier returning a WeatherFeather object, and it enables us to write reliable, fast and stable unit tests like the following:
@ExtendWith(MockitoExtension.class)
class ImprovedWeatherReporterTest {
@Mock
WeatherFetcher weatherFetcher;
ImprovedWeatherReporter sut;
@BeforeEach
void setUp() {
sut = new ImprovedWeatherReporter(() -> weatherFetcher);
}
@Test
void producesReports() throws IOException {
when(weatherFetcher.fetchTemperature("NewYork")).thenReturn(71);
when(weatherFetcher.fetchWeather("NewYork")).thenReturn("Sunny");
when(weatherFetcher.fetchTemperature("Montreal")).thenReturn(68);
when(weatherFetcher.fetchWeather("Montreal")).thenReturn("Cloudy");
String actual = sut.generateWeatherReport(Arrays.asList("NewYork", "Montreal"));
assertThat(actual).isEqualTo("=== Weather Report ===\n" +
"NewYork: Sunny, 71F\n" +
"Montreal: Cloudy, 68F\n");
}
@Test
void exception() throws IOException {
when(weatherFetcher.fetchTemperature("Tokyo")).thenThrow(new IOException("catch this"));
String actual = sut.generateWeatherReport(Collections.singletonList("Tokyo"));
assertThat(actual).isEqualTo("=== Weather Report ===\n" +
"Tokyo: Failed to fetch data\n");
}
}First, we create a mock of WeatherFetcher and inject a Supplier which returns the mock. With that, we can have complete control of the WeatherFetcher object the ImprovedWeatherReporter class relies on. Now we can specify what the WeatherFetcher object returns for what parameter with the mock framework you use. It will even enable us to test a case where the IOException is thrown, which was impossible with the tightly-coupled version of the WeatherReporter class. Just having a simple abstraction layer between your business logic and an external service makes writing unit tests much easier.
Another solution you might want to check is using a dependency injection framework to maintain singletons. You can easily make an object singleton and let the framework inject the singleton to the objects the framework maintains. It will reduce a lot of boilerplates, and some of the code I introduced here will be unnecessary. Especially if your code base has a dependency injection framework already, I recommend checking the documentation of your framework. For Google Guice, check this out: https://github.com/google/guice/wiki/Scopes
Conclusion
We’ve seen a couple of common idioms that can be used to apply the Singleton Pattern in Java, and discussed a common maintainability issue that is sometimes caused by applying the pattern. When you see code where there are a lot of getInstance() calls in the middle of business logic, it might be good to take some time to see if you would be able to run your business logic in isolation for unit tests. If your code was unit test friendly, your code would be likely to have wider coverage of tests and it would make future maintenance easier and the code base more stable.