Threads in Java

History  Process is a program in execution  Has stack/heap memory  Has a program counter  Multiuser operating systems since the sixties  OS schedules process A  Process A runs until it is interrupted  OS saves process A’s state  OS schedules process B  Process A has what is necessary to execute  OS interrupts process B, saving its state  OS schedules process A  Most programming languages do not have techniques to permit the user to specify concurrent activities  User wanting to program concurrently used operating system calls  ADA, developed for defense applications, was the first language to permit user-level concurrency  Java has tools that permit concurrency  threads, so-called lightweight processes (because they lack their own address space)  These are portable across platforms

Uses  Single-threaded applications can lead to long delays  E.g., a process waiting for i/o must get what it needs until it can resume  While that process is waiting, no other process can run  Example  User is downloading an audio clip  If single threaded, user must wait for the clip to be downloaded before he/she can listen.  If multi-threaded, one thread does download another plays back. The threads are synchronized so that listening does not begin until there is something to listen to.  Multithreading is difficult and error-prone.

Thread States  New: thread has been created  Runnable: executing its task  Waiting: waiting for another thread to perform a task  Timed Waiting: sleeping for a specific amount of time  Blocked: waiting for resources  Terminated: thread is complete

Examples  Timed Waiting  Word processor that periodically saves work  If thread did not sleep, it would have to be in a continuous loop, asking the main process if it’s time to back up  consumes resources  Blocked  Thread issues i/o request.  Thread cannot resume until the request has been fulfilled.  Runnable  Collapses two o/s states: ready and running

Scheduling  The JVM schedules threads by priority  The scheduling algorithm used depends on the os  All multiuser os support some form of time-slicing  Many possibilities  Round robin  Multi-level priority queue (p. 1049)

Implementing Threads Directly  Implement the Runnable interface  Method run() contains the code that the thread will execute  Examples: ThreadCreator/PrintTask

Using the Executor/Service Interfaces  Executor  Manages the execution of Runnable objects  Collects a group of threads into a thread pool  Reuses existing threads to eliminate the overhead involved in creating threads  Optimizes the number of threads to ensure that processor stays busy  ExecutorService  Interface that extends Executor and adds additional methods to manage thread lifecycle  Examples: TaskExecutor/PrintTask

Synchronization  Issue  Multiple threads of control  Single Address space  Imagine that we have two control structures  Parbegin  initializes parallel processing  Parend  ends parallel processing  Look at the following scenario

Withdraw(){ Record acctRec; parbegin{paulProc();heidiProc();}} ATM Example

Parallel Processes paulProc(){ read request; if (acctRec.bal >= request) { issue request; acctRec.bal -= request; acctRec.bal -= request; }}paulProc(){ read request; if (acctRec.bal >= request) { issue request; acctRec.bal -= request; acctRec.bal -= request; }}

Critical Section  The part of the program where shared memory is accessed

Race Condition  Two or more processes/threads reading/writing shared data  Result depends on who gets there first

Mutual Exclusion  Mechanism to prevent more than one process from entering its critical section  Need for such a mechanism was recognized in the sixties  Most languages do not provide program- level mutual exclusion facilities  Programmer must rely on operating system calls

Monitor  Java’s solution to the mutual exclusion problem  Every object has a monitor and a monitor lock  Monitor ensures that its object’s lock is held by a maximum of one thread at a time.  If a thread holds a lock, other threads trying to access the same object will be blocked until the thread is released.

High Level  Synchronized statement  Specifies that a thread must hold a monitor lock to execute the synchronized statements.  Monitor allows only one thread at a time to execute statements within a synchronized block  When a thread finishes executing the synchronized statements, the lock is released. Synchronized ( object ) {statements}

Producer/Consumer Problem  Class IPC problem  There is a buffer shared by  A producer who produces objects  A producer who consumes objects  Simplest approach: strict alternation  Consumer must not consume before producer produces  Producer must not produce object B before object A has been consumed