How to load the entire compiled class from a folder?

Question

I have folder operators. In this folder, I compiled the file (one AND 4 interface statement that implements the statement). The goal is to download the entire .class file from this folder and use it in the main program. I use these entries:

    File operatorFile = new File("D:\\operators");
    URL operatorFilePath = operatorFile.toURL();          
    URL[] operatorFilePaths = new URL[]{operatorFilePath};
    ClassLoader operatorsLoader = new URLClassLoader(operatorFilePaths);

 //Plus,Minus,Multiply,Divide are classes that implement operator interface
   Class[] operatorClass = new Class[]{ operatorsLoader.loadClass("Plus"), operatorsLoader.loadClass("Minus"),operatorsLoader.loadClass("Multiply") , operatorsLoader.loadClass("Divide") };

Then I use this element to call the methods of the .class class:

Method methodsInOperator;
Object instance;
String operatorSign;

for(Class operatorCls : operatorClass)
{
   instance = operatorCls.newInstance();
    methodsInOperator = operatorCls.getMethod("getSign", null); 
    operatorSign = (String)methodsInOperator.invoke(instance, null);
                    if(operatorSign.equals(elementInExpression[2]))
                    {
    methodsInOperator = operatorCls.getMethod("calculate", new Class[] { double.class, double.class } ); 
                        output =(double)methodsInOperator.invoke(instance, firstNumber, secondNumber);  
                    }
                }

But below the statute dose does not work dynamically, and if we put another .class file in the folder, then the folder program stops working.

Class[] operatorClass = new Class[]{ operatorsLoader.loadClass("Plus"), operatorsLoader.loadClass("Minus"),operatorsLoader.loadClass("Multiply") , operatorsLoader.loadClass("Divide") };

My goal is to dynamically load all classes and check whether they implement the operator and, in accordance with the getSing () method, select the best class. Can anyone help me?

Answer 1

There are two parts:

• .
• , /.

, - .

public Class[] getOperators(File operatorFile) throws MalformedURLException, ClassNotFoundException {
    ClassLoader operatorsLoader = new URLClassLoader(new URL[] { operatorFile.toURI().toURL() });

    File[] files = operatorFile.listFiles(new FilenameFilter() {
        @Override public boolean accept(File dir, String name) {
            return name.endsWith(".class");
        }
    });
    ArrayList<Class> operators = new ArrayList<>();
    for (File file : files) {
        String className = file.getName().substring(0, file.getName().length() - 6);
        Class<?> clazz = operatorsLoader.loadClass(className);
        if(OperatorInterface.class.isAssignableFrom(clazz)) {
            operators.add(clazz);
        }
    }
    return operators.toArray(new Class[operators.size()]);
}

, , OpeartorInterface. - , , .

Answer 2

SPI- :

• , ,

java -cp "main.jar; " my.package.MainClass

2. "META-INF/services"

3. , . , Operation com.calc "com.calc.Operation"

4. . , . . "com.calc.Addition", 4- "com.calc.Devision" ..

5. ServiceLocator :

   ServiceLoader<Operation> operations = ServiceLoader.load(Operation.class);
   Iterator<Operation> ite = operations.iterator();
   while (ite.hasNext()) {
       Operation op = ite.next();
       System.out.println("operation implementation is: " + op.getClass());
       Number result = op.calc(3,4);
       System.out.println("Result of 3 and 4 is " + result);
   }
   

SPI https://docs.oracle.com/javase/tutorial/sound/SPI-intro.html , ServiceLocator Java 6. SPI, .

, /META-INF/services

Answer 3

If I were you, I would give the Reflections library a chance. This is a very nice program that allows you to do exactly what you want:

Reflections reflections = new Reflections(new ConfigurationBuilder()
    .setUrls(ClasspathHelper.forPackage("my.project.prefix"))
    .setScanners(new SubTypesScanner()),
    .filterInputsBy(
        new FilterBuilder().includePackage("my.project.prefix")));

where my.project.prefixpoints to your folder.

Then you can simply use the instance reflectionsto get your classes:

Set<Class<? extends Operator>> operators = 
    reflections.getSubTypesOf(Operator.class);

Here Operatorwill be your base class or interface.