Unit 04

Question 1

bytecode

Answer 1

The ‘machine code’ of the Java Virtual Machine, i.e. the result of compiling a Java program.

Question 2

bytecode verifier

Answer 2

The part of the Java Virtual Machine that checks whether the bytecode about to be executed has not been tampered with.

Question 3

class loader

Answer 3

A part of the Java Virtual Machine that loads classes at runtime into the Java Virtual Machine’s memory. Java developers can implement their own specialised class loaders.

Question 4

conditionally thread-safe class

Answer 4

A class that requires some external synchronisation to achieve thread safety. For example, individual accesses may not need external synchronisation, but a sequence of them might.

Question 5

contention

Answer 5

A situation in which multiple threads are attempting to obtain the same resource.

Question 6

countdown latch

Answer 6

A flexible thread-coordination mechanism that makes one group of threads wait for each of N other threads to reach a certain point in their execution, with N being the initial value of the latch’s counter.

Question 7

divide-and-conquer strategy

Answer 7

A problem-solving strategy that breaks down a problem into subproblems of the same or similar type, continuing the process until the obtained subproblems are simple enough to be solved directly.

Question 8

external synchronisation

Answer 8

Synchronisation done by the callers of methods.

Question 9

factory method

Answer 9

A method that creates objects. Typically, a factory method is a static method that hides the concrete class of the returned object: the client knows only which interface the object implements.

Question 10

fail-fast operation

Answer 10

An operation that fails as soon as it detects some violation to consistency invariants. For example, iterators over pre-Java 1.5 collections throw an exception as soon as they detect that another thread is modifying the collection.

Question 11

garbage collection

Answer 11

A runtime technique to free the memory occupied by objects that are no longer used. The Java Virtual Machine specification does not impose any specific garbage collection algorithm.

Question 12

green thread

Answer 12

A thread that is not visible to the operating system and has to be created, scheduled and terminated by the Java Virtual Machine.

Question 13

immutable class

Answer 13

A thread-safe class whose instances never change once they have been created.

Question 14

internal synchronisation

Answer 14

Synchronisation done by the methods themselves.

Question 15

interpreter

Answer 15

A program that executes another program.

Question 16

Java Virtual Machine (JVM)

Answer 16

A virtual machine defined mainly with the execution of Java programs in mind. The Java Virtual Machine is a specification, and there are many implementations of it, optimised for different platforms or Java editions.

Question 17

just-in-time (JIT) compilation

Answer 17

A technique to compile a sequence of virtual machine instructions into machine code as the sequence is being executed.

Question 18

master–slave model

Answer 18

A model of communication in which one device or process (the master) has unidirectional control over other devices or processes (the slaves). For example, in a client–server system, the client is the master and the server is the slave.

Question 19

memory barrier

Answer 19

A special instruction to make local and shared memories consistent with each other.

Question 20

memory model

Answer 20

A specification of how and when updates to shared data have to be propagated across different memories and processes.

Question 21

native thread

Answer 21

A thread visible to the operating system and scheduled by it.

Question 22

nested monitor lockout

Answer 22

A situation in which a thread T blocks or waits while holding a lock on the monitor that would enable other threads to make T progress. Thread T has therefore locked itself inside the monitor, keeping the ‘key’, i.e. the monitor’s lock.

Question 23

priority queue

Answer 23

A queue that orders elements according to their associated priority so that the next element to be removed from the queue has the highest priority.

Question 24

scalability under contention

Answer 24

The ability of an algorithm to maintain a high throughput under contention, i.e. to keep a high ratio of number of threads per time unit that access the same resource.

Question 25

thread pool

Answer 25

A set of threads, each one taking a task from a common queue, executing it and then fetching the next task. The main aim of the pool is to reuse threads for multiple tasks, in order to optimise the memory usage and the performance of the application.

Question 26

thread safety

Answer 26

The property of a class behaving under concurrent access in the same way as under sequential execution. There are various levels of thread safety, depending on how much external synchronisation is necessary to achieve it.

Question 27

thread-compatible class

Answer 27

A class that provides no internal synchronisation. As such, access to it requires external synchronisation within a concurrent application.

Question 28

thread-hostile class

Answer 28

A class that is not safe to use in concurrent programs. No amount of external synchronisation will make instances of such classes behave as if they were accessed sequentially.

Question 29

thread-safe class

Answer 29

A class that has mutable instances and where the effect of any concurrent access to an instance could also be obtained by some sequential execution. Thread-safe classes do not require any external synchronisation.

Question 30

virtual machine (VM)

Answer 30

An abstract computing device implemented in software. A virtual machine defines a certain set of instructions and how code and data are organised in the machine’s memory.

Question 31

volatile variable

Answer 31

A variable declared with the volatile modifier. For practical purposes it is as if such variables are always accessed directly in shared memory. Moreover, an access to a volatile variable creates a memory barrier.

Question 32

weakly consistent operation

Answer 32

An operation that does not guarantee consistency invariants and as such does not fail under concurrent accesses. The iterators returned by the concurrency classes introduced in Java 1.5 are weakly consistent and never throw a exception due to concurrent modification.

Question 33

wrapper method

Answer 33

A method that adds new functionality or properties to existing objects. The Collections class provides static wrapper methods to turn thread-compatible collections into conditionally thread-safe ones.