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Java - Scanner (System.in) and "Blocking Thread"

2 main questions:

  • When you make a scanner from System.in and then write, say, "scan.nextLine ()", what actually happens before you hit enter? Is the thread running the code asleep and only wakes up when a newline appears in System.in? How else will it wait for human input?

  • When a thread is blocked, does that mean it is sleeping? Or does it just mean that it cannot enter a certain piece of code, but can it do other things?

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At any given time, each thread is in the same thread state. Possible thread states :

  • NEW

    : A thread that has not yet started is in this state.
  • RUNNABLE

    : A thread running in the Java Virtual Machine is in this state.
  • BLOCKED

    : A thread blocked waiting for a monitor lock is in this state.
  • WAITING

    : A thread that is waiting indefinitely for another thread to perform a certain action is in this state.
  • TIMED_WAITING

    : A thread is in this state, waiting for another thread to perform an action before the specified timeout.
  • TERMINATED

    : The exit from this thread is in this state.

The only state in which a thread executes any code is state RUNNABLE

.



Reading from System.in

while there is not enough data, the stream will be put into state BLOCKED

. There it will wait for more data or closure InputStream

, whichever comes first (not sure if System.in

it can be closed, but possibly other threads), and then it will again RUNNABLE

.

Technically, sleeping ( TIMED_WAITING

) and blocking ( BLOCKED

) are not the same states, but either of these states will cause the thread to wait for something before continuing. The difference is that threads BLOCKED

wait for some I / O operation (success or failure), while TIMED_WAITING

threads wait for a signal from some other thread in the JVM or for a given time, whichever comes first.

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