Java review-basic6
1. Weak references：
In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector, unlike a strong reference. An object referenced only by weak references – meaning "every chain of references that reaches the object includes at least one weak reference as a link" – is considered weakly reachable, and can be treated as unreachable and so may be collected at any time. Some garbage-collected languages feature or support various levels of weak references
个人理解：
weak reference不能像强引用那样，保护自己不被回收，所以可以像unreachable的类型一样随时被回收。
2.JINI
Jini (pronounced DJEE-nee like the Arabic word for "magician") is a network architecture concept that Sun Microsystems calls "spontaneous networking." Using Jini, users will be able to plug printers, storage devices, speakers, and any kind of device directly into a network and every other computer, device, and user on the network will know that the new device has been added and is available. Each pluggable device will define itself immediately to a network device registry. When someone wants to use or access the resource, their computer will be able to download the necessary programming from it to communicate with it. No longer will the special device support software known as adevice driver need to be present in an operating system. The operating system will know about all accessible devices through the network registry.
个人理解：
JINI就是一个系统可以加入其它的设备，只要他存在device registry
3.
JMS： It is a messaging standard that allows application components based on the Java Enterprise Edition (Java EE) to create, send, receive, and read messages. It allows the communication between different components of a distributed application to be loosely coupled, reliable, and asynchronous.
JMS provider: An implementation of the JMS interface for a Message Oriented Middleware (MOM). Providers are implemented as either a Java JMS implementation or an adapter to a non-Java MOM.
JMS client: An application or process that produces and/or receives messages.
JMS producer/publisher: A JMS client that creates and sends messages.
JMS consumer/subscriber: A JMS client that receives messages.
JMS message: An object that contains the data being transferred between JMS clients.
JMS queue: A staging area that contains messages that have been sent and are waiting to be read (by only one consumer). Contrary to what the name queue suggests, messages don't have to be received in the order in which they were sent.
A JMS queue: only guarantees that each message is processed only once.
JMS topic: A distribution mechanism for publishing messages that are delivered to multiple subscribers.
个人理解：
It's a messaging system and Java component can create, send, recive and read message form this system, we can send string type or serlized object.
3.JGroups：
JGroups is a reliable multicast system written in the Java language.
JGroups adds a "grouping" layer over a transport protocol, internally keeping a list of participants. This list is used to:
•Make the application aware of the listeners
•Make some or all transmissions reliable
•Allow totally ordered transmissions
JGroups is a toolkit for reliable multicast communication. It can be used to create groups of processes whose members can send messages to each other. JGroups enables developers to create reliable multipoint (multicast) applications where reliability is a deployment issue. JGroups also relieves the application developer from implementing this logic themselves. This saves significant development time and allows for the application to be deployed in different environments without having to change code.
个人理解：
It's a layer add over transport protocol in order ot multicast communication.
