56.1 Concurrency Basics
Concurrency is the concept of executing two or more tasks at the same time (in parallel). Tasks may include methods (functions), parts of a program, or even other programs. With current computer architectures, support for multiple cores and multiple processors in a single CPU is very common.
The Java Platform has always offered support for concurrent programming, which was the basis for implementing many of the services offered by Java EE containers. At Java SE 5, additional high-level API support for concurrency was provided by the java.util.concurrent package.
Prior to Java EE 7, there were no specific APIs that allowed enterprise developers to use concurrency utilities in a safely standard manner. The Java EE web and EJB containers instantiate objects using container-managed thread pools. Therefore, using Java SE concurrent APIs to instantiate Thread objects was strongly discouraged. If a developer creates a new (non-managed) Thread object, the container could not guarantee that other Java EE platform services (for example, transactions and security) would be part of this Thread.
56.1.1 Threads and Processes
The two main concurrency concepts are processes and threads.
Processes are primarily associated with applications running on the operating system (OS). A process has specific runtime resources to interact with the underlying OS and allocate other resources, such as its own memory, just as the JVM process does. A JVM is in fact a process.
The Java programming language and platform are primarily concerned with threads.
Threads share some features with processes, since both consume resources from the OS or the execution environment. But threads are easier to create and consume many fewer resources than a process.
Because threads are so lightweight, any modern CPU that has a couple of cores and a few gigabytes of RAM can handle thousands of threads in a single JVM process. The precise number of threads will depend on the combined output of the CPU, OS, and RAM available, as well as on correct configuration (tuning) of the JVM.
Although concurrent programming solves many problems and can improve performance for most applications, there are a number of situations where multiple execution lines (threads or processes) can cause major problems. These situations include the following:
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Deadlocks
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Thread starvation
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Concurrent accessing of shared resources
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Situations when the program generates incorrect data
