Determinando quando uma Thread Termina

1. Métodos para verificar se uma thread terminou:

isAlive()

• Retorna true se a thread ainda estiver em execução; caso contrário, retorna false.
• Usado para verificar continuamente o estado de threads.

join()

• Faz com que a thread que chamou o método espere até que a thread especificada termine.
• Existem variações que permitem definir um tempo máximo de espera.

2. Exemplo usando isAlive():

// Verifica se as threads estão vivas
class MyThread implements Runnable {
    Thread thrd;

    MyThread(String name) {
        thrd = new Thread(this, name);
        thrd.start(); // Inicia a thread
    }

    public void run() {
        System.out.println(thrd.getName() + " starting.");
        try {
            for (int count = 0; count < 10; count++) {
                Thread.sleep(400); // Suspende a execução por 400ms
                System.out.println("In " + thrd.getName() + ", count is " + count);
            }
        } catch (InterruptedException exc) {
            System.out.println(thrd.getName() + " interrupted.");
        }
        System.out.println(thrd.getName() + " terminating.");
    }
}

class MoreThreads {
    public static void main(String[] args) {
        System.out.println("Main thread starting.");
        MyThread mt1 = new MyThread("Child #1");
        MyThread mt2 = new MyThread("Child #2");
        MyThread mt3 = new MyThread("Child #3");

        // Verifica se as threads ainda estão vivas
        do {
            System.out.print(".");
            try {
                Thread.sleep(100);
            } catch (InterruptedException exc) {
                System.out.println("Main thread interrupted.");
            }
        } while (mt1.thrd.isAlive() || mt2.thrd.isAlive() || mt3.thrd.isAlive());

        System.out.println("Main thread ending.");
    }
}

Pontos importantes:

• A thread principal verifica continuamente com isAlive() até que todas as threads filhas terminem.
• A execução das threads é agendada pelo Java, então a saída exata pode variar.

3. Exemplo usando join():

// Aguarda o término das threads com join()
class MyThread implements Runnable {
    Thread thrd;

    MyThread(String name) {
        thrd = new Thread(this, name);
        thrd.start(); // Inicia a thread
    }

    public void run() {
        System.out.println(thrd.getName() + " starting.");
        try {
            for (int count = 0; count < 10; count++) {
                Thread.sleep(400); // Suspende a execução por 400ms
                System.out.println("In " + thrd.getName() + ", count is " + count);
            }
        } catch (InterruptedException exc) {
            System.out.println(thrd.getName() + " interrupted.");
        }
        System.out.println(thrd.getName() + " terminating.");
    }
}

class JoinThreads {
    public static void main(String[] args) {
        System.out.println("Main thread starting.");
        MyThread mt1 = new MyThread("Child #1");
        MyThread mt2 = new MyThread("Child #2");
        MyThread mt3 = new MyThread("Child #3");

        try {
            mt1.thrd.join(); // Aguarda Child #1 terminar
            System.out.println("Child #1 joined.");
            mt2.thrd.join(); // Aguarda Child #2 terminar
            System.out.println("Child #2 joined.");
            mt3.thrd.join(); // Aguarda Child #3 terminar
            System.out.println("Child #3 joined.");
        } catch (InterruptedException exc) {
            System.out.println("Main thread interrupted.");
        }

        System.out.println("Main thread ending.");
    }
}

Saída esperada (pode variar):

Main thread starting.
Child #1 starting.
Child #2 starting.
Child #3 starting.
In Child #1, count is 0
In Child #2, count is 0
In Child #3, count is 0
...
Child #1 terminating.
Child #1 joined.
Child #2 terminating.
Child #2 joined.
Child #3 terminating.
Child #3 joined.
Main thread ending.

Pontos importantes:

• join() garante que a thread principal só continue após a conclusão das threads filhas.
• A saída mostra o encerramento de cada thread na ordem esperada.