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Java 3: Odds & Ends
Advanced Programming
Techniques
javac options
-deprecation to get deprecation
information
-d to specify the destination for class
files
-verbose for more detailed output
-O for optimization
java options
Use –D to specify properties
Some often used properties:

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http.proxyHost
http.proxyPort
Use –X to specify non-standard
properties:

-Xms<size>
set initial Java heap size
See java documentation for more info
Security manager
-Djava.security.manager
installs the default security manager (very
restrictive)
-Djava.security.policy
specifies the policy file
Use policytool utility to generate policy
files
jar
Use jar utility to create jar files
Command format like tar
File format like zip, but includes a
manifest file MANIFEST.MF
Jar files can be made executable by
adding a Main-Class line to the manifest
javadoc
javadoc utility generates an HTML
documentation, API, with all the
classes’ information
Uses the code and the special javadoc
comments
/** starts the special doc comment
*/ ends it
javadoc continued
@author – author name (for class)
@param – parameter name and description
(method)
@return – description of return value
(method)
@exception – exception name and description
@deprecated – an item is obsolete and
shouldn’t be used (class, method, or variable)
javadoc example
package stars.ui;
import java.awt.*;
/** * A Viewer for text.
*
* @author Thomas Funck
* @version 1.0
*
* @see Viewer
*/
public class TextViewer extends Frame implements Viewer {
/** The TextArea used to display text */
private TextArea text;
/** Creates a new TextViewer */
public TextViewer() {... }
/** Sets the object to be viewed.
* @param o the object to be viewed
* @exception ClassCastException thrown when the specified object
* is not of type String */
public void setObject(Object o) throws ClassCastException { … }
Java threads
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Use the built-in Thread class in Java.
You need to supply the programming that
the thread will follow in order to do multithreaded programming.
There are two ways to get a new thread in
Java
 Extend (subclass) the Thread class. Provide a
run method in the subclass.
 Create a Thread object incorporating an object
that specifies the Thread’s run method.
extend (subclass) the built-in
Thread class
In file AThread.java:
class AThread extends Thread {
public AThread(String name) {super(name);}
public void run() {
System.out.println(“My name is” + getName());
}
}
In file test.java:
class test {
public static void main(String[] args) {
AThread a = new AThread(“mud”); // Thread name
a.start();
}
}
implement Runnable interface
In file AClass.java:
class AClass implements Runnable {
public void run() {
System.out.println(“My name is “ +
Thread.currentThread().getName());
}}
In file test2.java:
class test {
public static void main(String[] args){
AClass aObject = new AClass();
// Use alternative constructor for Thread class
Thread aThread = new Thread(aObject, “mud, too”);
// Thread name
aThread.start();
}}
Extend (inherit) Thread versus
implement Runnable
The “implementing Runnable”
technique allows you to extend another
class as well as getting threads.
Since Java does not have full multiple
inheritance, you can’t do that with the
first method.
Thread itself implements Runnable
Thread Construction
Different Thread constructors accept
combinations of arguments supplying:
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A Runnable object, in which case
Thread.start invokes run of the
supplied Runnable object
A String that serves as an identifier. Not
useful except for tracing and debugging
ThreadGroup in which the new Thread
should be placed
Starting threads
Invoking its start method causes an
instance of class Thread to initiate its run
method
A Thread terminates when its run method
completes by either returning normally or
throwing an unchecked exception
Threads are not restartable, even after they
terminate
isAlive returns true if a thread has been
started by has not terminated
More Thread methods
Thread.currentThread returns a
reference to the current Thread
Thread.sleep(long msecs) causes
the current thread to suspend for at
least msecs milliseconds
Thread.interrupt is the preferred
method for stopping a thread (not
Thread.stop)
Priorities
Each Thread has a priority, between
Thread.MIN_PRIORITY and
Thread.MAX_PRIORITY (from 1 to 10)
Each new thread has the same priority as the
thread that created it
The initial thread associated with a main by
default has priority Thread.NORM_PRIORITY (5)
getPriority gets current Thread priority,
setPriority sets priority
A scheduler is generally biased to prefer running
threads with higher priorities (depends on JVM
implementation)
The “run queue” of runnable
threads
The Java language specification does not
specify how Java is supposed to choose the
thread to run if there are several runnable
threads of equal priority.
One possibility – pick a thread and run it until
it completes, or until it executes a method
that causes it to move into a non-running
state.
Another possibility – “time slicing”: pick a
thread and run it for a short period of time.
Then, if it is not finished, suspend it and pick
another thread to run for the same period of
time.
The semantics of yield()
public static void yield()
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Causes the currently executing thread object to
temporarily pause and allow other threads to
execute.
Java Language Reference:
“In some cases, a large number of threads could be
waiting on the currently running thread to finish
executing before they can start executing. To
make the thread scheduler switch from the current
running thread to allow others to execute, call the
yield() method on the current thread. In order for
yield() to work, there must be at least one thread
with an equal or higher priority than the current
thread..”
Thread synchronization
t.wait() blocks until t.notify() or
t.notifyAll() method is called
Thread States and Scheduling
A Java thread can be in new, runnable,
running, suspended, blocked,
suspended-blocked and dead.
The Threads class has methods that
move the thread from one state to
another.
Thread/process states
start
yield
stop
0
sleep/suspend resume
1
2 run
stop/end
3
suspend
dispatch
stop
stop
1 – terminated
3 – suspended
2 – running
4 - runnable
4
Thread states
New state – a Thread newly created.
Runnable – after being started, the Thread
can be run. It is put into the “run queue” of
Threads and waits its turn to run. “Runnable”
does not mean “running”.
Running – the thread is executing its code.
On a uniprocessor machine, at most one
thread can run at a time.
More thread states
Blocked – the thread is waiting for something
to happen
It is waiting for an i/o operation it is
executing to complete
It has been told to sleep for a specified
period of time through the sleep method
It has executed the wait() method and will
block until another thread executes a notify()
or notifyAll() method.
It will return to runnable state after sleeping,
notifying, etc.
More thread states
Dead
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The final state. After reaching this state
the
Thread can no longer execute.
A thread can reach this state after the run
method is finished, or by something
executing its stop() method.
Threads can kill themselves, or a thread
can kill another thread.
Java’s synchronized feature
Each Java object has a lock associated with it.
A method can be declared as synchronized
public synchronized void methodA(…) {…}
Only one thread can run a synchronized
method of an object at a time. The lock is
set when a thread starts execution of the
synchronized method. A thread blocks if the
lock is currently held by some other thread
running a synchronized method of the object.
Static methods versus
ordinary methods
For synchronized static methods, Java obtains
a lock for the class before executing the
method.
For synchronized non-static methods, Java
obtains a lock for the object before executing
the method.
Java can synchronize on an
object, too.
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synchronized (expression) statement;
“expression” is something that must
resolve to an object or an array. There will
be an internal lock associated with that
object or array, and the thread must obtain
a lock on it before it will execute the
statement, which is regarded as a critical
section of code.
An example of synchronization
on objects
public static void SortIntArray( int[] a) {
synchronize(a) {
// do array sort here… gets a lock on the
array a so some other thread can’t change the
values
// in a while we’re trying to do the sorting
}
}