Java 2 Threads

Front 1

What are the two different things that a "thread" in Java means?

Back 1

1. An instance of class java.lang.Thread
2. A thread of execution

Front 2

What is an "instance" of Thread?

Back 2

It is just an object. It has variables and methods, and lives and dies on the heap.

Front 3

What is a thread of execution?

Back 3

An individual process (a "lightweight" process) that has its own call stack. In Java, there is one thread per call stack and in reverse, one call stack per thread.

Front 4

What schedules the threads in Java?

Back 4

The JVM operates like a mini-OS and schedules its own threads regardless of the underlying operating system.

Front 5

List the 4 methods in the Thread class that you need to know for the exam?

Back 5

start()
yield()
sleep()
run()

Front 6

How do you start a thread of execution, a new call stack?

Back 6

By invoking "run()"

Front 7

What are the two ways you can define and instantiate a thread?

Back 7

1. Extend the java.lang.Thread class
2. Implement the Runnable interface

Front 8

Why isn't extending the Thread class not a good OO practice?

Back 8

Because subclassing should be reserved for classes that extend an existing class. So you should only do this when you have a more specialized version of a Thread class (i.e you have more specialized thread-specific behavior).

Front 9

In the real world, which way would you most likely define and instantiate a thread?

Back 9

By implementing the Runnable interface

Front 10

If you extend java.lang.Thread how would you define code to run in a separate thread?

Back 10

1. Extend the Thread class
2. Override the "run()" method

Front 11

What is the limitation of extending the Thread class?

Back 11

You can't extend anything else.

Front 12

In extending the Thread class can you overload the "run()" method and if you do when will it be called?

Back 12

Yes you can overload it but it won't run unless you call it - it will not be used as the basis of a new call stack.

Front 13

Every thread of execution begins as an instance of class _____.

Back 13

Thread

Front 14

How do you instantiate a Thread if you just extended the Thread class?

Back 14

MyThread t = new MyThread();

Front 15

How do you instantiate a Thread if you implement Runnable?

Back 15

1. First you instantiate your Runnable class:
My Runnable r = new MyRunnable();
2. Next, you get yourself an instance of java.lang.Thread to run your job and give it your job:
Thread t = new Thread(r);

Front 16

When do you use the no-arg constuctor for Thread and when do you call the one-arg constructor for the Thread?

Back 16

1. no-arg when you extend Thread - then the thread will run its own "run()" method.
2. one-arg (you pass in your Runnable) when you implement Runnable - then the new thread will run your "run()" method.

Front 17

Can you pass a single Runnable instance to multiple Threads?

Back 17

Yes - this means several threads of execution will be running the very same job.

Front 18

Give the 7 constructors we care about for the Thread class.

Back 18

1. Thread()
2. Thread(Runnable target)
3. Thread(Runnable target, String name)
4. Thread(String name)
5. Thread(ThreadGroup group, Runnable target)
6. Thread(ThreadGroup group, Runnable target, String name)
7. Thread(ThreadGroup group, String name)

Front 19

What state is a thread in when it has been instantiated but not started and is it alive?

Back 19

The "new" state. No, it is not yet alive.

Front 20

How can you test the "aliveness" of a thread - that it has been started but has not yet completed its "run()" method?

Back 20

By calling the "isAlive()" method on the Thread instance.

Front 21

How do you start a new Thread?

Back 21

You call start "start()" on a Thread instance:
t.start();

Front 22

What happens when you call "start()"?

Back 22

1. A new thread of execution starts (with a new call stack)
2. The thread moves from the "new" state to the "runnable" state.
3. When the thread gets a chance to execute, its target "run()" method will run

Front 23

What do you start, a Thread or a Runnable?

Back 23

a Thread

Front 24

What call stack does a "run()" method go on if it is called directly?

Back 24

It goes onto the current call stack rather than at the beginning of a new call stack - this is legal although it does not start a new thread of execution.

Front 25

Since the Runnable instance doesn't have a reference to the Thread instance, how do we get the thread name?

Back 25

First invoke the static "Thread.currentThread()" method, which returns a reference to the currently executing thread, and then invoke "getName()" on that returned refernce:
Thread.currentThread().getName()

Front 26

Does a thread have a name if you don't explicitly give it a name?

Back 26

Yes, it still has a name

Front 27

If you don't give a thread a name what is the form of the name and what is the name of the main thread?

Back 27

The name of the main thread is "main". The form of a thread name is like "Thread-0".

Front 28

What is the only thing that is guaranteed with Java threads?

Back 28

Each thread will start, and each thread will run to completion. How this happens is not just JVM dependent but also runtime dependent. Order of threads running, a thread executing until it's done and a loop completing before another thread begins is not guaranteed.

Front 29

What is in charge of when threads are run?

Back 29

the scheduler

Front 30

When is a thread done being a thread?

Back 30

When its target "run()" method completes.

Front 31

What happens when a thread completes its "run()" method?

Back 31

The thread ceases to be a thread of execution, the stack for that thread dissolves, and the thread is considered dead.

Front 32

Once a thread is dead, is it gone?

Back 32

No, it's still a Thread object, just not a thread of execution.

Front 33

What method can't you call on a dead Thread instance?

Back 33

its "start()" method

Front 34

What does the thread scheduler do?

Back 34

It is part of the JVM and decides which thread should run at any given moment, and also takes threads out of the run state.

Front 35

What does it mean for a thread to be eligible to run?

Back 35

It is in the runnable state

Front 36

In a single processor mahcine, how many threads can run at a time?

Back 36

one

Front 37

Is the order in which runnable threads are chosen to run guaranteed?

Back 37

No

Front 38

Give the signatures of 4 methods from the java.lang.Thread class that influence thread scheduling.

Back 38

public static void sleep(long millis) throws Interrupted Exception
public static void yield()
public final void join()
public final void setPriority(int newPriority)

Note: sleep() and join() have overloaded versions not shown above

Front 39

Give the signatures of 3 methods from the java.lang.Object class that influence thread behavior.

Back 39

public final void wait()
public final void notify()
public final void notifyAll()

Note: the wait() method has 3 overloaded version including the one above

Front 40

List the 5 states that a thread can be in.

Back 40

new, runnable, running, waiting/blocked/sleeping, dead

Front 41

What is the "new" state of a thread?

Back 41

The state the thread is in after the Thread instance has been instantiated but the "start()" method has not been invoked on the thread. It is a live Thread object but not yet a thread of execution. It is "not alive".

Front 42

What is the "runnable" state of a thread?

Back 42

The state of a thread when it is eligible to run, but the scheduler has not selected it to be the running thread. A thread enters this state when the "start()" method is invoked, but it also returns to this state after either running or coming back from a blocked, waiting, or sleeping state. It is "alive".

Front 43

What is the "running" state of a thread?

Back 43

This is the state when the scheduler selects it (from a runnable pool) to be the currently executing process.

Front 44

What is the one thing that "waiting/blocked/sleeping" have in common?

Back 44

The thread is still alive, but is currently not eligible to run, although it might return to a runnable state later if a particular event occurs.

Front 45

Can one thread tell another thread to block?

Back 45

No

Front 46

What is the "dead" state of a thread?

Back 46

A thread is considered dead when its "run()" method completes.

Front 47

What happens when you invoke "start()" on a dead thread?

Back 47

You'll get a runtime(not compiler) exception because a dead thread can never be brought back to life.

Front 48

What method would you use to "slow a thread down"?

Back 48

the static Thread.sleep() method.

Front 49

What exception can the "sleep()" method throw?

Back 49

a checked InterruptedException

Front 50

What does the time specified in "sleep()" mean?

Back 50

It is the minimum duration in which the thread won't run. There is not a guarantee that the thread will start running again as soon as the time expires and the thread wakes.

Front 51

Why can't "sleep()" be used for one thread to put another thread to sleep?

Back 51

Because it is a static method - so it can only put the currently running thread to sleep.

Front 52

Threads usually run with some ____ usually as a number between 1 and __.

Back 52

priority
10

Front 53

What type of scheduling is used in most JVMs?

Back 53

preemptive, priority-based scheduling

Front 54

Do all JVMs use time slicing?

Back 54

No

Front 55

In most JVMs, how does the scheduler use thread priorities?

Back 55

The running thread will be of equal or greater priority than the hightest priority threads in the pool.

Front 56

How should you use thread priorities in your program?

Back 56

As a way to improve the efficiency but don't depend on thread-scheduling priority for correctness.

Front 57

What is the default priority of a thread?

Back 57

the priority of the thread of execution that created it

Front 58

How can you yourself set a thread's priority?

Back 58

By calling "setPriority()" on a Thread instance.

Front 59

Give the three constants (static final variables) in the Thread class that define the range of thread priorities.

Back 59

Thread.MIN_PRIORITY (1)
Thread.NORM_PRIORITY (5)
Thread.MAX_PRIORITY (10)

Front 60

What does "yield()" do?

Back 60

It might - and often does - make a running thread give up its slot to another runnable thread of the same priority, there's no guarantee - the yielding thread might just be chosen again over all the others.

Front 61

What does a call to "join()" do?

Back 61

Guaranteed to cause the current thread to stop executing until the thread it joins with (in other words, the thread it calls "join()" on) completes. If the thread it's trying to join with is not alive, however, the current thread won't need to back out.

Front 62

Besides the calls to "sleep()", "yield()", and "join", give three other ways in which a thread might leave the running state?

Back 62

1. The thread's "run()" method completes
2. A call to "wait()" on an object
3. A thread can't acquire the lock on the object whose method code it's attempting to run.

Front 63

What is an "atomic" operation?

Back 63

The operation, regardless of the number of actual statements (or underlying byte code instructions), is completed before any other thread code that acts on the same data.

Front 64

What can you guarantee for an "atomic" operation?

Back 64

Even if the thread running the atomic operation moves in and out of the runnable state, no other running thread will be able to act on the same data.

Front 65

What are the two things you must do to protect data?

Back 65

1. Mark the variables "private"
2. Synchronize the code that modifies the variables

Front 66

How does synchronization work?

Back 66

With locks. Every object in Java has a built-in lock that only comes into play when the object has synchronized method code. Since there is only one lock per object, if one thread has picked up the lock, no other thread can enter the synchronized code until the lock is released.

Front 67

Only _____ can be synchronized, not _____.

Back 67

methods
variables

Front 68

How many locks does an object have?

Back 68

just one

Front 69

Can a class have both synchronized and nonsynchronized methods?

Back 69

Yes

Front 70

If two methods are synchronized in a class, can one thread be accessing one of the methods and another by accessing the other method?

Back 70

No

Front 71

If a class has both synchronized and nonsynchronized methods, can multiple threads still access the nonsynchronized methods?

Back 71

Yes

Front 72

What happens with a thread's locks when it goes to sleep?

Back 72

It takes them with it.

Front 73

Can a thread acquire more than one lock? Give examples

Back 73

Yes
A thread can enter a synchronized method, thus acquiring a lock, and then immediately invoke a synchronized method on a different object, thus acquiring that lock as well.

Front 74

Can a thread acquire a lock and then attempt to call a synchronized method on that same object?

Back 74

Yes - the JVM know that this thread already has the lock for this object so is free to call other synchronized methods on the same object using the lock the thread already has.

Front 75

Can you just synchronize a block of code rather than a method?

Back 75

Yes, we call this a "synchronized block"

Front 76

When you synchronize a block of code, what lock do you use?

Back 76

You specify which object's lock you want to use as the lock. This gives you the ability to have more than one lock for code synchronization within a single object.

Front 77

Can static methods be synchronized and if so what lock is used?

Back 77

Yes, there is only one copy of the static data you're trying to protect, so you only need one lock per class to synchronize static methods - a lock for the whole class and every class loaded in Java has a corresponding instance of java.lang.Class representing that class and it's this instance whose lock is used to procted the static methods of the class, if they're synchronized.

Front 78

If a thread tries to enter a synchronized method and the lock is already taken, what happens?

Back 78

The thread is said to be blocked on an object's lock. The thread goes into a kind of pool for that particular object ahd has to sit there until the lock is released and the thread can again become runnable/running.

Front 79

How does a deadlock happen?

Back 79

It occurs when two threads are blocked, with each waiting for the other's lock. Neither can run until it gives up the lock, so they'll sit forever and ever and ever ...

Front 80

What is one design strategy that can be used to help avoid deadlock?

Back 80

strategies for always acquiring locks in a predetermined order.

Front 81

What 3 methods does java.lang.Object have that help threads communicate about the status of an event that the threads care about?

Back 81

wait(), notify(), and notifyAll()

Front 82

What kind of context must "wait()
, "notify()", and "notifyall()" be called from?

Back 82

a synchronized context. A thread can't invoke a wait or notify method on an object unless it owns that object's lock.

Front 83

What happen when a thread executes "wait()"?

Back 83

Every object can have a list of threads that are waiting for a signal (notification) from the object. A thread gets on this waiting list by executing the "wait()" method of the target object. From that moment, it doesn't execute any further instructions until the "notify()" method of the target object is called.

Front 84

What happens to a lock when a thread waits?

Back 84

It temporarily releases the lock for other threads to use, but it will need it again to continue execution.

Front 85

What happens when a thread calls "wait()" does not own the lock?

Back 85

It will throw an IllegalMonitorStateException which is not a checked exception so you don't have to catch it explicitly.

Front 86

What is the second form of "wait()"?

Back 86

It accepts a number of milliseconds as a maximum time to wait. If the thread is not interrupted, it will continue normally whenever it is notified or the specified timeout has elapsed.

Front 87

What happens with the lock when "notify()" is called?

Back 87

It doesn't mean the thread gives up its lock at that moment. If the thread is still completing synchronized code, the lock is not released until the thread moves out of synchronized code.

Front 88

What happens when you call "notifyAll()"?

Back 88

All of the threads that are waiting will be notified and start competing to get the lock. As the lock is used and released by each thread, all of them will get into action without a need for further notification.

Front 89

What method is defined in interface Runnable?

Back 89

run()

Front 90

Which 2 methods in the Thread class are static and which are non-static?

Back 90

sleep() and yield() are static
start() and join are non-static