So, if i have understood this correctly, a thread goes into waiting state when we call wait on an object and it goes into blocked state when it is waiting for a lock on an object(like when trying to get into a synchronized block or method).
How does I/O-methods like read() put a Thread in a blocked state though? I understand WHY it has to be in a blocked state, waiting for data that it can read but i'm also interested in HOW. How does the JVM notify the thread that it can continue when data in the resource its trying to read, is available again?
It doesn't change the state of the thread to BLOCKED
public static void main(String[] args) throws IOException {
Thread main = Thread.currentThread();
new Thread(() -> {
for (int i = 0; i < 10; i++) {
System.out.println(main + " is in "+main.getState()+" state");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
throw new AssertionError(e);
}
}
}).start();
System.in.read();
}
prints
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
Thread[main,5,main] is in RUNNABLE state
instead the OS doesn't return from the read until there is some data and the OS decides whether and when to context switch the thread/process.
How does the JVM notify the thread that it can continue when data in the resource its trying to read, is available again?
The OS wakes the thread when there is more data or the stream has been closed. The JVM doesn't get involved.