Quiz 3 Flashcards
JDBC
Java Data Base Connectivity. An API that consists of libraries of classes used to interact with a relational database from a java application
JDBC API allows you to access databases from just about any database vendor (T/F)
True
JDBC API lets you create a Connection object which allows you to log into the database (T/F)
True
JDBC API lets you create a Statement object to send queries/update statements to the database source (T/F)
True
JDBC API lets you create a PreparedStatement object that can be cached and executed only once (T/F)
False. It can be executed multiple times.
JDBC API lets you use a CallableStatement for executing stored procedures on the database (T/F)
True
JDBC API lets you process the results returned from the DB in a Collection object (T/F)
False. It is returned in a ResultSet object.
JDBC API lets you close the connection to the DB (T/F)
True
JDBC API looks after all of the translation work needed for your java app and the database to talk to each other (T/F)
True
What is needed to run a JDBC API?
A JDBC compatible database driver which must be loaded by your java application code in order to be used. Applications can load and use more than one driver depending on the back-end DB.
What are the steps to access a database via a Java app?
- Load JDBC driver
- Create Connection object and pass in DB URL, user and pass as args
- Create Statement/Prepared Statement object
- Execute the query/stored procedure
- Process the results
- Close ResultSet (if used)
- Close Statement object
- Close the Connection object
Types of DB Drivers
- JDBC to ODBC bridge (inefficient, unreliable)
- Native-API partly Java
- JDBC to Network protocol pure Java
- Native protocol pure Java (most efficient)
Model View Controller (MVC)
Design pattern that helps you break code into functional segments with clear boundaries as to what each segment does (compound design pattern)
MVC architecture
Design GUI using three separate code entities:
- Model - maintain the data to be displayed in GUI
- View - displayes data contained in model
- Controller - handles user inputs and updates model
If you have a view registered as a listener for the model then it can automatically update itself when the event fires (T/F)
True
Multiple views can be serviced by the same model (T/F)
True
TableModel
Interface that specifies 9 methods to manipulate data from a ResultSet:
- addTableModelListener
- getColumnClass
- getColumnCount
- getColumnName
- getRowCount
- getValueAt
- isCellEditable
- removeTableModelListener
- setValueAt
Process
- Can be thought as the entire program
- Every process has its own address space
- Process based multitasking means 2+ programs can be run concurrently
- Process based multitasking is controlled by the OS
Thread
- A small part of a process
- All threads in a process share the address space
- Thread based multitasking means a process can run 2+ threads concurrently
- Thread based multitasking is under the control of the JVM
Option 1 for creating threads in Java
Have the class being run as a thread implement the Runnable interface. Runnable represents a job that can execute concurrently with other jobs. Coder must only implement the run() method.
Option 2 for creating threads in Java
Extend the class from the Thread class, which also implements the Runnable interface, and override the Thread class run() method with your own code.
run() is automatically invoked by the JVM but should still be called explicitly (T/F)
False. Coder should not call run() explicitly, otherwise the code will run inside the current thread, no new thread will be used.
What does the start() method do?
Signals the JVM that a thread is ready and waiting to run, so it joins the queue of threads waiting to run.
Stop(), suspend() and resume() are unstable and should not be used (T/F)
True
What does the yield() method do?
Causes the thread to be polite and give up its remaining time to allow another thread to run.
What does the sleep() method do?
Safely and temporarily stop a thread (used instead of suspend() ). Needs to be put inside a try-catch.
What are the constant thread priorities?
MIN = 1 NORM = 5 MAX = 10
Round-Robin Scheduling
If all runnable threads have equal priority then each gets an equal time on the CPU.
What is thread contention / thread starvation?
When a lower priority thread may never get to run if there is a higher priority thread or if an equal priority thread never yields.
To avoid contention coders must ensure higher priority threads invoke sleep() / yield() periodically (T/F)
True
What is the Lost Update Scenario? (Concurrent Processing)
When two users try to modify and access the same data and one was unable to complete the process in one time slice.
What was done to prevent Lost Update?
Transaction rules were created. When a transaction is started, the row gets locked to prevent other users from accessing it.
Synchronized Data Structure
Puts locks in place when a thread accesses an element of the vector and blocks other threads from accessing it until the first thread completes its transaction.
Access to values in an arrayList structure that is synchronized is faster and safe to use anytime (T/F)
False. They are only safe to use in single thread applications.
Race Condition
If thread access is not synchronized either by the coder or some feature of the data structure which can cause data corruption. Occurs when several threads try to access the same data during their time slice.
Thread-Safe
A class or data structure that will not permit race conditions to occur.
Coders must prevent more than one thread at a time from entering the critical region of the program where variable / data structure is accessed in order to prevent race conditions (T/F)
True
Most methods with a return value will not be corrupted by multi thread access (T/F)
False. Basically any method can be corrupted.
Lock Interface
Defines lock(), unlock(), and newCondition() and allows creation of locks with specified fairness policies.
True Fairness Policy
The longest waiting thread will be the first to obtain the lock and do business.
True fairness policy may result in slower performance but prevents thread starvation where a thread is unable to obtain a lock and cannot carry out its task (T/F)
True
Atomic classes can perform their operations atomically, meaning the whole operation is completed as a unit and cannot be interrupted (T/F)
True
What are the Atomic classes?
AtomicInteger, AtomicBoolean, AtomicLong, AtomicReference
AtomicInteger Class
Contains several methods which treat read-calculate-write as one atomic task, such that all steps are completed without interruptions (thread-safe).
Why not synchronize every method?
- Time cost (runs slowly)
2. Defeats the purpose of multi-threading (every thread has to wait for the method to be available)
Volatile
Applied to a variable. Does not do any locking. Informs the JVM that the variable will be accessed concurrently by multiple threads
For variables changed using R-U-W steps, volatile will prevent possible lost updates (T/F)
False. It will not prevent lost updates and should not be relied upon to do so.
Thread States
- New
- Runnable
- Running
- Blocked
- Terminated/Dead
Calling yield() does not automatically result in a thread giving up its time (T/F)
True
wait() is a method of the Thread class (T/F)
False. It is from the Object class.
How to stop a thread properly?
Call interrupt() rather than stop()
