AccessController Sınıfı

Giriş
Şu satırı dahil ederiz.

import java.security.AccessController;

doPrivileged Neden Lazım?

Yani Security Manager yoksa doPrivileged() yapmaya gerek yok.

Without an installed Security Manager you don't need a privileged block. However, if you are writing fully general library code, which may be executed with a security manager in place, and the caller of the library may not have the needed permissions, then without a PrivilegedAction your code will be denied access as well, ...

Açıklaması şöyle

There are lot of operations in Java that require the caller domain to have certain permissions for successful execution of those operations. System.getProperty is one of those operations. All file related operations also need special permissions. When you use AccessController.doPrivileged to invoke those operations, the operation is executed with all the rights(permissions) of your protection domain.

doPrivileged metodu - PrivilegedAction

Exception fırlatmıyor

Örnek

Şöyle yaparız

class PasswordManager {
 
  public static void changePassword() {
    FileInputStream fin = openPasswordFile();
    if (fin == null) {
      // No password file; handle error
    }
  }
 
  private static FileInputStream openPasswordFile() {
    final String password_file = "password";
    final FileInputStream fin[] = { null };
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {
          try {
            // Sensitive action; can't be done outside
            // doPrivileged() block
            fin[0] = new FileInputStream(password_file);
          } catch (FileNotFoundException x) {
            // Report to handler
          }
          return null;
        }
    });
    return fin[0];
  }
}

doPrivileged metodu - PrivilegedExceptionAction
PrivilegedActionException fırlatıyor. Açıklaması şöyle. 

Örnek - File

Belli bir dizine erişmek için şöyle yaparız.

Reader reader;
try {
  reader = AccessController.doPrivileged(new PrivilegedExceptionAction<Reader>() {
    public Reader run() throws IOException {
      return new FileReader(file);
    }
  });
} catch (PrivilegedActionException e) {
  throw (IOException) e.getCause();
}

Örnek - File

Şöyle yaparız

String jarPath = ...;
URL jarUrl = new File(jarPath).toURI().toURL();
try (URLClassLoader classLoader = AccessController.doPrivileged(
  (PrivilegedAction<URLClassLoader>) () ->
    new URLClassLoader(new URL[]{jarUrl}, MyMain.class.getClassLoader())
)) {
  Class<?> clazz = loadMainClass(classLoader, mainClass);

  Method main = getMainMethod(clazz, calledByMember);
  String[] args = new String[] {""}
  main.invoke(null, args);
}

Örnek - File

Şöyle yaparız

private static void loadLibrary(final String libName) {
  AccessController.doPrivileged(new PrivilegedAction<Void>() {
    public Void run() {
      System.loadLibrary(libName);
      return null;
    }
  });
}

private static void loadLibraryFile(final String libFileName) {
  AccessController.doPrivileged(new PrivilegedAction<Void>() {
    public Void run() {
      System.load(libFileName);
      return null;
    }
 });
}

Örnek - Hiding Exceptions

Şöyle yaparız

public static void changePassword() throws FileNotFoundException {
  //Use own privilege to open the sensitive password file
  final String password_file = "password";
  try {
    final InputStream in = AccessController.doPrivileged(
      new PrivilegedExceptionAction<InputStream>() {
        @Override   
        public InputStream run() throws FileNotFoundException {
          return openPasswordFile(password_file); //call the privileged method here
        }
      }
	);
  //Perform other operations such as password verification
  } catch (PrivilegedActionException exc) {
    Exception cause = exc.getException();
    if (cause instanceof FileNotFoundException) {
      throw (FileNotFoundException)cause;
    } else {
      throw new Error("Unexpected exception type",cause);
    }
  }
}

RMI Registry Arayüzü

Giriş
Şu satırı dahil ederiz

import java.rmi.registry.Registry;

bind metodu - String + Remote
Örnek ver

rebind metodu - String + Remote
Örnek ver

IOException Sınıfı

Giriş
Dosya yok, dizin yok, dizin yaratamadım gibi hatalar fırlatmak için kullanılır.

@SuppressWarnings Anotasyonu

Örnek
Şöyle yaparız.

@SuppressWarnings("UnstableApiUsage")

Örnek
Şöyle yaparız.

@SuppressWarnings("serial")
public class Foo implements Serializable {
  ...
}

Thread Sınıfı

Giriş
Her thread "native thread" olarak hayat bulur. Açıklaması şöyle

Each time you start a thread the JVM needs to perform syscalls to allocate the thread stack memory segment and the "red zone" memory segment, and initialize them. (The default thread stack size is typically 500KB or 1MB.) Then there are further syscalls to create the native thread and schedule it.

constructor - default
run metodu override edilmelidir. Şöyle yaparız.

Thread thread = new Thread() {
  @Override
  public void run () {
    ...
  }
};
thread.start ();

Ya da kalıtım ile yaparız.

public class MyThread extends Thread {
  @Override
  public void run() {
    super.run();
      ...
  }
}

constructor - string
Run metodu override edilmelidir. Şöyle yaparız.

static class MyThread extends Thread {
  public MyThread(String s) {
    super(s);
  }
  @Override
  public synchronized void run() {
    ...
  }
}

Şöyle yaparız.

MyThread thread = new MyThread("thread0");

constructor - Runnable
Örnek
Şöyle yaparız.

public class MyTask implements Runnable {
  public void run () {...}
}

Thread thread = new Thread (new MyTask()));

Örnek
Lambda ile şöyle yaparız.

new Thread( () -> {
    ...
}).start();

Örnek
Şöyle yaparız.

Thread thread = new Thread(() -> {...});
thread.start();

constructor - Runnable + string
Şöyle yaparız.

Thread thread = new Thread(() -> {...}, "my thread");

currentThread metodu
Şöyle yaparız.

Thread t = Thread.currentThread();

dumpStack metodu
Standart Stream'e çıktıyı verir.
Örnek
Şöyle yaparız.

Thread.dumpStack()

getAllStackTraces metodu
Şöyle yaparız. Bir map döner. Key olarak Thread sınıfı kullanılır.

System.out.println("Running thread count : " + 
 Thread.getAllStackTraces().keySet().size());

Tüm thread'lerin stack trace çıktısını almak için şöyle yaparız.

Thread.getAllStackTraces()
  .keySet()
  .stream()
  .map(Thread::getStackTrace)
  .map(Arrays::asList)
  .forEach(list -> System.out.println(list.stream()
                                      .map(i -> i.toString())
                                      .collect(Collectors.joining("\n\t"))))

getDefaultUncaughtExceptionHandler metodu
Şöyle yaparız.

Thread.UncaughtExceptionHandler handler =
  Thread.getDefaultUncaughtExceptionHandler();

getId metodu
Şöyle yaparız.

System.out.println(Thread.currentThread().getId());

getName metodu
Şöyle yaparız.

System.out.println(t.getName());

getStackTrace metodu
Açıklaması şöyle

The last element of the array represents the bottom of the stack, which is the least recent method invocation in the sequence.

Şöyle yaparız.

StackTraceElement[] stackTraceElements = Thread.currentThread().getStackTrace()

getState metodu

Açıklaması şöyle

NEW 
A thread that has not yet started is in this state.
RUNNABLE 
A thread executing in the Java virtual machine is in this state.
BLOCKED 
A thread that is blocked waiting for a monitor lock is in this state.
WAITING 
A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
TIMED_WAITING 
A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
TERMINATED 
A thread that has exited is in this state.

Örnek
Şöyle yaparız.

System.out.println("State of thread : " + thread.getState());

Örnek
Eğer thread başlamadıysa başlatmak için şöyle yaparız.

if (thread.getState() == Thread.State.NEW) {
  thread.start();
}

Örnek
Şöyle yaparız.

if (thread.getState() == Thread.State.TIMED_WAITING) {
  ...
} else if (gameThread.getState() == Thread.State.TERMINATED) {
  ...
} else {
  ...
}

interrupt metodu
Açıklaması şöyle

If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.

Belirtilen metodlar üzerinde beklemekte olan thread'in içindeki içindeki "interrupt status" bayrağını indirir ve thread'in InterruptedException almasına sebep olur. Şöyle yaparız.

t.interrupt ();

interrrupted metodu - static
Thread ve Interrupt Kullanımı yazısına taşıdım

isAlive metodu
Açıklaması şöyle

A thread is alive if it has been started and has not yet died.

Şöyle yaparız.

while(thread.isAlive ()){...}

isInterrupted metodu
Thread ve Interrupt Kullanımı yazısına taşıdım

join metodu
Şöyle yaparız.

try {
  t.join ();
} catch (InterruptedException e) {
  ...
}

onSpinWait metodu
Java 9 ile geliyor.
Örnek
Şöyle yaparız.

else if ((LockSupport.nextSecondarySeed() & OVERFLOW_YIELD_RATE) == 0)
  Thread.yield();
else
  Thread.onSpinWait();
return 0L;

run metodu
Açıklaması şöyle

The Thread class itself implements Runnable, though its run method does nothing. An application can subclass Thread, providing its own implementation of run,

Eğer Thread nesnesi içine Runnable vermezsek aşağıdaki kod hiç bir şey yapmaz.

Thread t=new Thread();
t.start ();

Thread nesnesini kalıtmamız gerekir.

public class MyThread extends Thread {
  public void run () {...}
}

run vs start açıklaması şöyle

One tricky question I found in various scenarios is how a thread can be started and what is the difference between calling run() and start() methods on a thread instance.

Thread printTrehad = new Thread(() -> System.out.println("..."));

printThread.start();

printThread.run();

printThread.run();

Both method calls are doing the same, they are displaying a text on the console, but the difference is that by calling the start() method, a new thread will be started and will print the text, and if you call the run() method, the text will be displayed by the calling thread.

Another difference is that the run() method can be called multiple times, but the start() method cannot be invoked more than one(the second method call will throw IllegalStateException)

setDeamon metodu
Deamon Thread yazısına taşıdım.

setDefaultUncaughtExceptionHandler metodu - static
Örnek
Şöyle yaparız.

Thread.setDefaultUncaughtExceptionHandler (myHandler);

Handler şöyledir.

Thread.UncaughtExceptionHandler myHandler = new Thread.UncaughtExceptionHandler(){
  @Override
  public void uncaughtException(Thread thread, Throwable ex) {
    ...
  }
};

Handler içinde sanırım yapılabilecek en mantıklı şey exception'ın nereden geldiğini yazdırmak. Şöyle yaparız.

ex.printStackTrace ();

Örnek
Şöyle yaparız.

Thread.setDefaultUncaughtExceptionHandler((thread,t) -> {
  if(t instanceof OutOfMemoryError) {
    System.exit(1);
  }
});

setName metodu
Şöyle yaparız.

Thread t = new Thread(() -> {
  System.out.println("Thread name is " + Thread.currentThread().getName());
});
t.setName("Peter");
t.start();

setPriority metodu
Thread.setPriority yazısına taşıdım

setUncaughtExceptionHandler metodu
Thread Sınıfı ve Uncaught Exception yazısına taşıdım.

sleep metodu - milisaniye kullanır

Thread.sleep metodu yazısına taşıdım

start metodu
Örnek

Şöyle yaparız. Thread'i başlatır.

t.start ();

Örnek

Bitmiş thread tekrar başlatılamaz. Elimizde şöyle bir kod olsun. Bu kod IllegalThreadStateException fırlatır.

mythread.start();       
mythread.join();

mythread.start();   //IllegalThreadStateException
mythread.join();

stop metodu
Bu metod deprecated. Yani kullanılmamalı. Açıklaması şöyle

Why is Thread.stop deprecated?
Because it is inherently unsafe. Stopping a thread causes it to unlock all the monitors that it has locked. (The monitors are unlocked as the ThreadDeath exception propagates up the stack.) If any of the objects previously protected by these monitors were in an inconsistent state, other threads may now view these objects in an inconsistent state. Such objects are said to be damaged. When threads operate on damaged objects, arbitrary behavior can result. This behavior may be subtle and difficult to detect, or it may be pronounced. Unlike other unchecked exceptions, ThreadDeath kills threads silently; thus, the user has no warning that his program may be corrupted. The corruption can manifest itself at any time after the actual damage occurs, even hours or days in the future.

wait metodu
notify() veya interrupt() metodu çağrılıncaya kadar bekler. interrupt() metodu çağrıınca InterruptedException fırlatır. Bunu fırlatınca thread'in interrupted bayrağını temizler. Açıklaması şöyle.

Throws:
InterruptedException - if any thread interrupted the current thread before or while the current thread was waiting. The interrupted status of the current thread is cleared when this exception is thrown.

Örnek

Şöyle yaparız. Çıktı olarak false verir.

public static void main(String args[]) {
  Thread t = new Thread() {
    public void run() {
      try {
        synchronized (this) {
          wait();
        }
      } catch (Exception e) {
        e.printStackTrace();
        System.out.println(Thread.interrupted()); //false
      }
    }
  };
  t.start();

  t.interrupt();
  for (;;) {
  }

}

yield metodu
Açıklaması şöyle

Whenever a thread calls the Thread.yield() method, it gives a hint to the thread scheduler that it is ready to pause its execution. The thread scheduler is free to ignore this hint.

If any thread executes the yield method, the thread scheduler checks if there is any runnable (waiting to be executed) thread with same or high priority than this thread. If the processor finds any thread with higher or same priority then it will switch to a new thread. If not, the current thread keeps executing.