Why newInstance throws InstantiationException in my code?

Question

I have two process as shown below. Each of my process has run and shutdown method

Process processA = new ProcessA("processA", getProcessAProperties());
Process processB = new ProcessB("processB", getProcessBProperties());

• I want to have different thread pool configuration for each Process so that ProcessA run in its own thread pool and ProcessB run in its own thread pool independent of each other.
• And I cannot share Process object between each threads of its own thread pool.

Below is how my Process class looks like and my ProcessA, ProcessB class simply extends Process class. And I do all important stuff in my run method.

public abstract class Process implements Runnable {
  private Properties props;
  private String processName;

  public Process(String processName, Properties props) {
    this.processName = processName;
    this.props = props;
  }

  protected abstract void shutdown();

  protected abstract void run(String processName, Properties props);

  @Override
  public final void run() {
    run(processName, props);
  }

  public Properties getProps() {
    return props;
  }

  public void setProps(Properties props) {
    this.props = props;
  }

  public String getProcessName() {
    return processName;
  }

  public void setProcessName(String processName) {
    this.processName = processName;
  }
}

Below is a simple example of how I am running my ProcessA with its own thread pool. There are three threads and each thread gets its own ProcessA object to work on. Now I want to extend this in a more generic way so that it can work for both my process ProcessA and ProcessB.

public static void main(String[] args) {
  int numberOfThreads = 3;
  ExecutorService executor = Executors.newFixedThreadPool(numberOfThreads);

  final List<Process> processes = new ArrayList<>();
  for (int i = 0; i < numberOfThreads; i++) {
    // each thread works on different Process object
    Process processA = new ProcessA("processA", getProcessAProperties());
    processes.add(processA);
    executor.submit(processA);
  }

  Runtime.getRuntime().addShutdownHook(new Thread() {
    @Override
    public void run() {
      for (Process process : processes) {
        process.shutdown();
      } 
      executor.shutdown();
      try {
        executor.awaitTermination(5000, TimeUnit.MILLISECONDS);
      } catch (InterruptedException e) {
        e.printStackTrace;
      }
    }
  });
}

So to solve this problem in a more generic way, I created a Process handler as shown below:

public final class ProcessHandler {
  private final ExecutorService executorServiceProcess;
  private final List<Process> processes = new ArrayList<>();
  private final Thread shutdownHook = new Thread() {
    @Override
    public void run() {
      for (Process process : processes)
        process.shutdown();
      executorServiceProcess.shutdown();
    }
  };

  public ProcessHandler(Process process, int poolSize) {
    this.executorServiceProcess = Executors.newFixedThreadPool(poolSize);
    Runtime.getRuntime().addShutdownHook(shutdownHook);
    for (int i = 0; i < poolSize; i++) {
      try {
        // this line throws exception
        Process p = process.getClass().newInstance();
        p.setProcessName(process.getProcessName());
        p.setProps(process.getProps());
        processes.add(p);
        executorServiceProcess.submit(p);
      } catch (InstantiationException | IllegalAccessException e) {
        e.printStackTrace();
      }
    }
  }

  public void shutdown() {
    Runtime.getRuntime().removeShutdownHook(shutdownHook);
    shutdownHook.start();
    try {
      shutdownHook.join();
    } catch (InterruptedException ex) {
      Thread.currentThread().interrupt();
    }
  }
}

And this is the way my main method looks now:

public static void main(String[] args) {

    Process processA = new ProcessA("processA", getProcessAProperties());
    Process processB = new ProcessB("processB", getProcessBProperties());

    // processA will run with three threads in its own thread pool
    ProcessHandler processHandlerA = new ProcessHandler (processA, 3);
    // processB will run with two threads in its own thread pool
    ProcessHandler processHandlerB = new ProcessHandler (processB, 2);

    // now I can call shutdown on them
    processHandlerA.shutdown();
    processHandlerB.shutdown();
}

This line in my ProcessHandler class Process p = process.getClass().newInstance(); throws exception as:

java.lang.InstantiationException: com.david.test.ProcessA

I am not sure why InstantiationException is getting thrown?

Just a note: Each of these processes are kafka consumer and generally kafka consumer are not thread safe so that's why I have to create a new object every time and submit to executor.

Update:

This is my ProcessA class looks like:

public class ProcessA extends Process {
  private KafkaConsumer<byte[], byte[]> consumer;

  public ProcessA(String processName, Properties props) {
    super(processName, props);
  }

  @Override
  public void shutdown() {
    consumer.wakeup();
  }

  @Override
  protected void run(String processName, Properties props) {
    consumer = new KafkaConsumer<>(props);
    System.out.println("Hello World");
    // do all kind of important stuff here
  }
}

Answer 1

Do you a concrete class that extents your Process abstract class?

abstract classes can not be instantiated on their own, see: http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html

Perhaps try extending your abstract class with a concrete class and creating instances of your concrete class, you can still cast them as a Process object if needed.

Answer 2

the constructor in ProcessA is

 public ProcessA(String processName, Properties props);

so there doesn't exist a no-argument constructor like:

 public ProcessA();

and

 Process p = process.getClass().newInstance()

will invoke a no-argument construct method, which does not exist, so the exception will appear.

For more information, please take a look at https://docs.oracle.com/javase/tutorial/reflect/member/ctorInstance.html.