Back in the old days a computer had a single CPU, and was only capable of executing a single program at a time. Later came multitasking which meant that computers could execute multiple programs (AKA tasks or processes) at the same time. It wasn't really "at the same time" though. The single CPU was shared between the programs. The operating system would switch between the programs running, executing each of them for a little while before switching.
Along with multitasking came new challenges for software developers. Programs can no longer assume to have all the CPU time available, nor all memory or any other computer resources. A "good citizen" program should release all resources it is no longer using, so other programs can use them.
Later yet came multithreading which mean that you could have multiple threads of execution inside the same program. A thread of execution can be thought of as a CPU executing the program. When you have multiple threads executing the same program, it is like having multiple CPU's execute within the same program.
Mulithreading is even more challenging than multitasking. The threads are executing within the same program and are hence reading and writing the same memory simultanously. This can result in errors not seen in a singlethreaded program. Some of these errors may not be seen on single CPU machines, because two threads never really execute "simultanously". Modern computers, though, come with multi core CPU's. This means that separate threads can be executed by separate cores simultanously.
If a thread reads a memory location while another thread writes to it, what value will the first thread end up reading? The old value? The value written by the second thread? Or a value that is a mix between the two? Or, if two threads are writing to the same memory location simultanously, what value will be left when they are done? The value written by the first thread? The value written by the second thread? Or a mix of the two values written? Without proper precautions any of these outcomes are possible. The behaviour would not even be predictable. The outcome could change from time to time.
Multithreading and Concurrency in Java
Java was one of the first languages to make multithreading easily available to developers. Java had multithreading capabilities from the very beginning. Therefore, Java developers often face the problems described above. That is the reason I am writing this trail on Java concurrency. As notes to myself, and any fellow Java developer whom may benefit from it.
The trail will primarily be concerned with multithreading in Java, but some of the problems occurring in multithreading are similar to problems occurring in multitasking and in distributed systems. References to multitasking and distributed systems may therefore occur in this trail too. Hence the word "concurrency" rather than "multithreading".
This trail is still work in progress. Texts will be published whenver time is available to write them. Below is a list of the current texts in this trail. The list is also repeated at the top right of every page in the trail.