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Differences in Java and C#: protected

Java and C# are very similar languages so if you have to switch between the two it’s easy to overlook subtle differences. One of the tricky bits is the meaning of protected access modifier. 

In C#, if you mark a field with protected keyword it will be available to the class that owns it and to its derived classes. In Java access will be broader. Not only the owner and derived classes will be able to access the field but also all classes defined in the same package. In C# similar effect can be achieved by assigning protected internal access level. Member marked like that has access which is a union of internal (same assembly) and protected levels. Important thing to note is that concepts of Java package and C# assembly are not equivalent. C# assembly can span multiple namespaces and is related to physical unit (EXE, DLL) that keeps intermediate code and metadata. Package in Java is more similar to namespace in C# with key (not only) difference that it has an impact on accessibility…

Below are two projects that show protected access level differences between Java and C# (both are available in this GitHub repository). C# program was made in Visual Studio 2015 Community and targets .NET 4.5.2. Java program was made in IntelliJ IDEA 15 Community Edition and is set to use Java 8. Version of Java/.NET is not relevant and there’s nothing special in the projects - could’ve been easily done in notepad but isn’t it great that this days we can get such awesome IDEs for free? :) 

Java and C# projects... Click to enlarge...

Java

Base.java

package com.example;

public class Base {
    protected int someProtectedFiled = 123;

    public void testAccessInBaseClass() {
        // SAME BEHAVIOR IN JAVA AND C#
        // The class can access its own protected field
        System.out.println(someProtectedFiled);
    }
}

Derived.java

package com.example;

public class Derived extends Base {
    public void testAccessInDerivedClass() {
        // SAME BEHAVIOR IN JAVA AND C#
        // The class can access inherited protected field
        System.out.println(someProtectedFiled);

        // DIFFERENT BEHAVIOR IN JAVA AND C#!
        // Access to protected field is possible because classes are in the same package
        // (notice access through qualifier of type Base instead of Derived)
        System.out.println(new Base().someProtectedFiled);
        // In C# the field would have to be public or protected internal (otherwise CS1540 error is produced)
    }
}

NotDerived.java

package com.example;

public class NotDerived {
    public void testAccessInNotDerivedClass() {
        // DIFFERENT BEHAVIOR IN JAVA AND C#!
        // Access to protected field is possible because classes are in the same package
        System.out.println(new Base().someProtectedFiled);
        // In C# the field would have to be public or protected internal (otherwise CS0122 error is produced)
    }
}

DerivedInAnotherPackage.java

package com.example.another;

import com.example.Base;

public class DerivedInAnotherPackage extends Base {
    public void testAccessInDerivedClassFromAnotherPackage() {
        // SAME BEHAVIOR IN JAVA AND C#
        // The class can access inherited protected field even from another package
        System.out.println(someProtectedFiled);

        // SAME BEHAVIOR IN JAVA AND C#
        // For the below to work (instead of compilation error) the field would have to be public
        // (notice access through qualifier of type Base instead of DerivedInAnotherPackage):
        // System.out.println(new Base().someProtectedFiled);
    }
}

C#

Base.cs

namespace Protected
{
    public class Base
    {
        protected int someProtectedFiled = 123;

        public void TestAccessInBaseClass()
        {
            // SAME BEHAVIOR IN JAVA AND C#
            // The class can access its own protected field            
            System.Console.WriteLine(someProtectedFiled);
        }
    }
}

Derived.cs

namespace Protected
{
    public class Derived : Base
    {
        public void TestAccessInDerivedClass()
        {
            // SAME BEHAVIOR IN JAVA AND C#
            // The class can access inherited protected field           
            System.Console.WriteLine(someProtectedFiled);

            // DIFFERENT BEHAVIOR IN JAVA AND C#!
            // For the below to work (instead of CS1540 compilation error) the field would have to be public            
            // or protected internal (notice access through qualifier of type Base instead of Derived):
            // System.Console.WriteLine(new Base().someProtectedFiled);
        }
    }
}

NotDerived.cs

namespace Protected
{
    public class NotDerived
    {
        public void TestAccessInNotDerivedClass()
        {
            // DIFFERENT BEHAVIOR IN JAVA AND C#!
            // For the below to work (instead of CS0122 compilation error) the field would
            // have to be public or protected internal:
            // System.Console.WriteLine(new Base().someProtectedFiled);
        }
    }
}

DerivedInAnotherAssembly.cs

using Protected;

namespace AnotherAssembly // Namespace doesn't matter
{
    public class DerivedInAnotherAssembly: Base
    {
        public void TestAccessInDerivedClassFromAnotherAssembly()
        {
            // SAME BEHAVIOR IN JAVA AND C#
            // The class can access inherited protected field even from another assembly            
            System.Console.WriteLine(someProtectedFiled);

            // SAME BEHAVIOR IN JAVA AND C#
            // For the below to work (instead of CS1540 compilation error) the field would have to be public
            // (notice access through qualifier of type Base instead of DerivedInAnotherAssembly):
            // System.Console.WriteLine(new Base().someProtectedFiled);           
        }
    }
}

I hope that comments in the code make everything clear and this dry topic is exhausted... I've just orderd USB shield for my Arduino so if it works the next post will be about moving this thing with PlayStation controller :)

Update (2016-03-14):

Unfortunately I don’t have time to write the post mentioned above but the good news is that you definitely can use SparkFun USB Host Shield with USB Host Shield 2.0 library to steer servos with PS3 controller :) Here’s a short video of how it worked on my paintball turret. And here’s a clip of test circuit (hardware diagram).

[OoB] Controlling gun turret with Spring Boot REST service (Java/Arduino)

This is the fifth post in my little "Out of Boredom" series dedicated to hobby projects with Arduino. Previous posts were all .NET based:

Now it's time for a bit of Java!

I will show you how to use Spring Boot framework to quickly create RESTful web service that can receive JSON requests and send commands to Arduino. The aim is to control a servo and relay based gun turret such as this one:

Gun turret prototype... Click to enlarge...

Click here to get an idea of how such turret operates. The video shows a prototype based on ASG pistol. Don't worry, my aim is quite peaceful: I want to build paintball gun turret :)

I will not discuss any electronics setup or Arduino code in this post. Take a look at the articles linked above to see information about PC-Arduino communication, controlling servos and building relay based circuit... I assume that you know what Spring Boot is, but advanced knowledge is not required to follow this post.

Full code of the project is available on GitHub (includes both Java application and Arduino sketch)...

The easiest way to create basic working Spring Boot project is to use Spring Initializr. I started my project by filling the form like this:

Spring Initializr settings... Click to enlarge...

Note that Gradle is used to create (fat) Jar package, Java 1.8 is used and the only required dependency is Web. This setup will produce an application running on Tomcat Embedded so there's no need for installing any web server. The app has support for REST controllers so we will be able to handle JSON based communication in a clean way with very little effort...

 

Without further ado, here is the piece of code responsible for receiving messages from clients:

package springarduino;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class TurretController {
    private final ArduinoConnection arduino;

    @Autowired
    public TurretController(ArduinoConnection arduino) {
        this.arduino = arduino;
    }

    @RequestMapping(value = "turret/execute", method = RequestMethod.POST, consumes = "application/json")
    public TurretResponse executeTurretAction(@RequestBody TurretRequest request) {
        if (request.getPan() < 0 || request.getPan() > 180) {
            throw new IllegalArgumentException("Pan out of 0..180 range (" + request.getPan() + ")");
        }

        if (request.getTilt() < 0 || request.getTilt() > 180) {
            throw new IllegalArgumentException("Tilt out of 0..180 range (" + request.getTilt() + ")");
        }

        boolean sent = arduino.controlTurret(request.getPan(), request.getTilt(), request.isFire());
        if (!sent) {
            throw new RuntimeException("Command not sent :(");
        }

        return new TurretResponse(request.getId(), "Command sent :)");
    }
}

TurretController class has @RestController annotation and a public method marked with @RequestMapping. The mapping specifies that executeTurretAction method should be invoked whenever a client makes a POST request to .../turret/execute URL. Any HTTP client capable of sending POST with Content-Type="application/json" can communicate with such Spring controller method. It can be some desktop application or a simple HTML page with a bit of jQuery for example. I was using Postman Chrome App to prepare requests. In the next post I will describe how to communicate with such controller from smartphone that runs PhoneGap/AngularJS based application...

executeTurretAction methods expects one argument of type TurretRequest:

package springarduino;

public class TurretRequest {
    private int id;
    private int pan;
    private int tilt;
    private boolean fire;

    // public getters and setters hidden for brevity
}

If client sends JSON payload such as this:

{
    "id": "311",
    "pan": "111",
    "tilt": "99",
    "fire": "true"
}

Spring will take care of creating TurretRequest object. Our service method returns TurretResponse:

package springarduino;

public class TurretResponse {
    private int id;
    private String message;

    public TurretResponse(int id, String message) {
        this.id = id;
        this.message = message;
    }

    // public getters and setters hidden for brevity
}

If everything goes well, this kind of data will be sent back to client:

{
    "id": 19,
    "message": "Command sent :)"
}

You don't have to do anything special to make this happen. Spring chooses HttpMessageConverter implementation to create response in a format that is expected by the client. Another nice feature of our @RestController is the error handling. Let's say that client provides invalid value of tilt angle - this is what is sent back as response:

{
    "timestamp": 1424382893952,
    "status": 500,
    "error": "Internal Server Error",
    "exception": "java.lang.IllegalArgumentException",
    "message": "Tilt out of 0..180 range (222)",
    "path": "/turret/execute"
}

Such message is easy to handle in error callbacks (thanks to 500 HTTP status code) and contains useful properties such as message and exception.

Notice that arduino object of ArduinoConnection type is used inside executeTurretAction method. ArduinoConnecton is a Spring bean responsible for communicating with Arduino over serial port.

Our controller gets reference to proper object thanks to Spring's IoC container. TurretController constructor is annotated with @Autowired so Spring knows that ArduinoConnection object needs to be injected.

This is the class responsible for talking to Arduino:

package springarduino;

import gnu.io.NRSerialPort;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import java.io.DataOutputStream;

@Component
public class ArduinoConnection {
    private static final int MESSAGE_SEPARATOR = 255;

    private final Logger log = LoggerFactory.getLogger(this.getClass());

    @Value("${arduinoPortName}")
    private String portName;

    @Value("${arduinoBaudRate}")
    private int baudRate;

    private NRSerialPort serial;


    @PostConstruct
    public void connect() {
        log.info("ArduinoConnection PostConstruct callback: connecting to Arduino...");

        serial = new NRSerialPort(portName, baudRate);
        serial.connect();

        if (serial.isConnected()) {
            log.info("Arduino connection opened!");
        }
    }

    @PreDestroy
    public void disconnect() {
        log.info("ArduinoConnection PreDestroy callback: disconnecting from Arduino...");

        if (serial != null && serial.isConnected()) {
            serial.disconnect();

            if (!serial.isConnected()) {
                log.info("Arduino connection closed!");
            }
        }
    }

    public boolean controlTurret(int pan, int tilt, boolean fire){
        try {
            // Actual values sent to Arduino will be in proper unsigned byte range (0..255)
            byte[] message = new byte[]{(byte) pan, (byte) tilt, (byte) (fire ? 1 : 0), (byte) MESSAGE_SEPARATOR};

            DataOutputStream stream = new DataOutputStream(serial.getOutputStream());
            stream.write(message);

            log.info("Turret control message sent (pan={}, tilt={}, fire={})!", pan, tilt, fire);
            return  true;
        } catch (Exception ex) {
            log.error("Error while sending control message: ", ex);
            return false;
        }
    }
}

@Component annotation is there to show Spring that ArduinoConnection is a bean and as such Spring should take care of its lifecycle and usage as dependency. By default Spring creates beans in singleton scope. This is fine for us - we only need one such object. connect method is marked with @PostConstruct. This makes the method an initialization callback that gets invoked when ArduinoConnection object is created (it will happen when application is started). @PreDestroy is used on disconnect method to make sure that connection to serial port is released when the program is closed.

controlTurret method is the piece of code that is responsible for sending gun turret action request to Arduino. That method is used in TurretController.executeTurretAction, remember? it uses instance of NRSerialPort to communicate over serial port (gnu.io.NRSerialPort package makes it possible). It comes form NeuronRobotics/nrjavaserial library which is a fork of RXTX that greatly simplifies serial port access. nrjavaserial takes care of loading proper native library needed to access the port (it worked well on my Windows 7 x64). As stated before, I'm not going to discuss Arduino communication and microcontroller code in this post. I will just note that you don't have to worry about casting int to byte while message array is created. It's sad but Java doesn't have unsigned bytes so it will show (byte)MESSAGE_SEPARATOR (which is 255) as -1 while debugging, but a correct value will go over wire to Arduino. Take a look at this screen shot from Free Device Monitoring Studio (you can use this tool to check what data is sent to Arduino through serial port):

Bytes sent to Arduino... Click to enlarge...

Let's get back to ArduinoConnection class: portName and baudRate properties are marked with @Value annotations. This way it is very easy to take the settings from configuration file. All you have to do is to create a file named application.properties in /src/main/resources directory and values will be automatically loaded from config. Here's the content of application.properties file: 

server.address = 192.168.0.17
server.port = 8090

arduinoPortName = COM3
arduinoBaudRate = 9600

Apart from aforementioned settings for Arduino, there are two other elements, namely: sever.address and server.port. By default Spring Boot runs the application on localhost:8080. I've changed this to custom settings to make it easy to access the application from devices connected to my WiFi network... Accessing TurretController from such devices is the reason why I added following CorsFilter class to the project:

package springarduino;

import org.springframework.stereotype.Component;

import javax.servlet.*;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@Component
public class CorsFilter implements Filter {

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        HttpServletResponse response = (HttpServletResponse) res;
        response.setHeader("Access-Control-Allow-Origin", "*");
        response.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS");
        response.setHeader("Access-Control-Max-Age", "3600");
        response.setHeader("Access-Control-Allow-Headers", "X-Requested-With");
        chain.doFilter(req, res);
    }

    public void init(FilterConfig filterConfig) {}

    public void destroy() {}
}

Thanks to this Cross-Origin Resource Sharing filter it's easy to make ajax calls to executeTurretAction method without using some trickery like JSONP to circumvent same-origin policy restrictions.

And that's it! All interesting elements of the Java web app were discussed. Full code is accessible on GitHub. Since this is Gradle based project, running it is as easy as typing gradlew run. I've included Gradle Wrapper in the repo so you don't even need to have Gradle installed...