Multithreading

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Multithreading

• Threads Concept
• Creating Threads by Extending the Thread class
• Creating Threads by Implementing the Runnable
  Interface
• Controlling Threads and Thread Status
• Thread Groups
• Synchronization
• Creating Threads for Applets
• Case Studies

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Threads Concept
Multiple threads on multiple CPUs
Multiple threads sharing a single CPU
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Creating Threads by Extending the Thread class
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Example 13.1Using the Thread Class to Create and
Launch Threads

• Objective Create and run three threads
• The first thread prints the letter a 100 times.
• The second thread prints the letter b 100 times.
• The third thread prints the integers 1 through
  100.

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Example 13.1Using the Thread Class to Create and
Launch Threads, cont.
TestThread
Run
Click the Run button to access the DOS prompt
then type java TestThread
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Creating Threads by Implementing the Runnable
Interface
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Example 13.2Using the Runnabe Interface to
Create and Launch Threads

• Objective Create and run three threads
• The first thread prints the letter a 100 times.
• The second thread prints the letter b 100 times.
• The third thread prints the integers 1 through
  100.

Run
Click the Run button to access the DOS prompt
then type java TestRunnable
TestRunnable
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Controlling Threads and Thread States

• void run()
• Invoked by the Java runtime system to execute the
  thread. You must override this method and provide
  the code you want your thread to execute.
• void start()
• Starts the thread, which causes the run() method
  to be invoked. Called by the runnable object in
  the client class.
• static void sleep(long millis)throws
  InterruptedException
• Puts the runnable object to sleep for a
  specifiedtime in milliseconds.

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Thread States
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Synchronization
A shared resource may be corrupted if it is
accessed simultaneously by multiple threads. For
example, two unsynchronized threads accessing the
same bank account causes conflict.
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Example 13.3Showing Resource Conflict

• Objective create and launch 100 threads, each of
  which adds a penny to a piggy bank. Assume that
  the piggy bank is initially empty.

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Example 13.3, cont
PiggyBankWithoutSync
Run
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The synchronized keyword

• To avoid resource conflicts, Java uses the
  keyword synchronized to synchronize method
  invocation so that only one thread can be in a
  method at a time. To correct the data-corruption
  problem in Example 13.3, you can rewrite the
  program as follows

PiggyBankWithSync
Run
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Web Server example I

• A server class starts max number threads of
  request_handler that extends Thread class.

hconfig new httpd_config("httpd.conf")
if(!hconfig.isValid()) System.out.println("
Configuration file not correct") return
mime new mime_config(hconfig.getValue("TypesC
onfig")) mime.setDefault(hconfig.getValue("De
faultType")) s new server(hconfig,mime)
if(s.isReady()) s.run()
Server.run
for(i0 iltmaxthreadsi) request_handler
server_thread new request_handler(ss,i,hc,mc,l)
server_thread.start()
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Request_handler.run
Request_handler.reset
public void run() while(true)
reset() s get_socket()
if(snull) // got an error, //
return and kill thread return
set_streams() get_request()
process_request() send_reply()
close_socket()
private void reset() env.reset()
r_method null r_page
null r_http_version null r_address
"" r_agent "-" r_accept
null r_referer "-" r_ctype
null r_clength "0"
r_command "" error_code 0
script false filename
null scriptname null filelength
0 filestream null
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Request_handler.get_socket
private synchronized Socket get_socket()
try Socket incoming ss.accept()
return(incoming) catch(IOException
e) System.out.println(e)
return(null)
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Web Server example II

• A main server class waits for connections and
  creates new thread when a new request arrives.
• The ServerHandler extends thread class.

while( true )
Socket socket sSocket.accept()
new ServerHandler( socket, numThreadsCreated,
srmProp
).start()
ServerHandler.run
BufferedReader in new BufferedReader(
new InputStreamReader(

socket.getInputStream()))
request.readRequest(in) response
new HResponse(request,socket.getOutputStrea
m()) socket.close()