Download Introduction to Java, OO and IDEs

Threads
ICW Lecture 10
Tom Chothia
Last Time
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XML
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XPATH
This Lecture
• URLs
• A reminder of Sockets.
• Threads: running processes at the same
time on the same computer.
• They can be tricky to use.
Uniform Resource Locators
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Many Internet systems use URLs, e.g.
http://www.cs.bham.ac.uk/index.html
Protocol
Host
FilePath
This URL refers to the file “index.html” on
the host “www.cs.bham.ac.uk” which
should be accessed using http.
URLs aren't just for HTTP
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https://www.gmail.com/index.html
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ftp://ftp.funet.fi/pub/standards/RFC/rfc
959.txt
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rmi://68.45.12.7/process
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svn+ssh://svn.cwi.nl/projects
More complex URLs
http://www.cs.bham.ac.uk:3080/index.html
Port number
ftp://[email protected]/myfiles.txt
Username
Quiry
http://en.wikipedia.org/wiki/Uniform_Resource_Locator#Synt
ax
URL syntax
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The most general URL has the syntax:
protocol://username:password@domain:port
/filepathname?query_string#anchor
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All parts are options!
URLs in Java
The java.net.URL class does URLs in Java e.g.:
URL myURL = new URL
(“http://www.cnn.com/index.html);
myURL.getHost();
myURL.getPort();
URLs in Java
The URL knows the protocol therefore it can
connect for you:
InputStream =
myURL.openStream();
More info at:
http://java.sun.com/j2se/1.4.2/docs/api/java/
net/URL.html
Sockets
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A reminder: Code from Lecture 6.
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Only one connection at a time.
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For multiple connections you need to
use Threads.
Threads


A thread is a Java process that runs at
the same time as other processes.
Use threads for:



Speed on Multi-process machines.
When you want to handle lots of requests
at the same time.
When it's more “natural”.
Example: Chat Server
• Servers handle the requests from the clients
concurrently: we can use Threads.
• Define the object that handles the request as
“runnable” and define a run method.
• Declare it as a new Thread.
• Call the start method on it.
Threads and Speed
• Threads only speed up your program if
your computer has more than one
processor.
• To find the number of processors:
Runtime runtime = Runtime.getRuntime();
int noOfProcessors =
runtime.availableProcessors();
Can “value” ever go below 0?
Class Counter {
private static int value = 1000;
public static void less (int i) {
if (i<value) {value=value – i; }
}
}
What about now?
Class Counter extends Thread {
private static int value = 1000;
public static void less (int i) {
if (i<value) {value=value – i; }
}
}
Synchronisation
• Threads run concurrently
• Threads share the same data space
• Threads can interact in unexpected ways e.g.
• The state of object can only be kept consistent
by guarding against such interactions.
What must we do to cope with
this?
• Parts of the program must declare that
they must have exclusive access to an
object whilst performing an operation
• synchronised is used to mark
methods or statements as needing
exclusive access
• This implements a lock on the object
Synchronised methods
• Use synchronized to declare critical
method
• Only one thread can executing a
synchronised method on an object at a
time:
– To execute a synchronised method the thread
must obtain the lock on that object
– Whilst it holds the lock, no other thread can
execute a synchronised method on that object - it
is blocked
What about now?
Class Counter extends Thread {
private static int value = 1000;
public static synchronised void
less (int i) {
if (i<value) {value=value – i; }
}
}
Synchronised Statements
• Individual statements can also be
synchronised.
• Syntax:
synchronized (object) {statement}
e.g:
...;
synchronized (foo)
{
...
i=foo.getSize();
foo.setSize(i*2); }
Deadlock
• Deadlock describes a state where the
program cannot proceed because of
interactions between locks held by
threads.
• For instance:
– A is blocked waiting for a lock held by B
– B is blocked waiting for a lock held by A
• The interactions can be indirect
Synchronisation/locks
• It is sometimes necessary to restrict access
to objects by multiple threads - to maintain
consistency
• As a rule you should minimise the use of
locks
• Problems can arise:
– not making code thread-safe
– deadlocks
• Such problems are hard to debug
Sleep
• A thread can choice to stop running for a
time using the sleep method:
• Thread.sleep(1000)
• Pauses the current Thread for about 1
sec.
Wait, Notify
• The wait() method cause the process to
stop and give up its locks.
• The process remains paused until
another process calls notify() on the
same object.
• Notifyall() restarts all waiting processes.
Communication Between
Threads
• Threads don't return values.
• Threads often run independenly to offer a
service.
• However a thread can write to e.g. a
buffer, for other threads to read from.
Main Program Creates a Buffer
Main Program
Results Buffer
Main can read from this buffer
Main Program
Results Buffer
Main creates a thread and
passes it the buffer
Main Program
Thread 1
Results Buffer
Main creates and starts
another thread
Main Program
Thread 1
Thread 2
Results Buffer
The threads write to the buffer,
from which Main can read
Main Program
Thread 1
Thread 2
Results Buffer
Conclusion
• Threads let you run concurrent
processes.
– Good for multi-core machines.
– Good for services.
• Extend Thread/Implenement Runable
• Define a run() method.
Next Time
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Javascript