不能含有静态（全局）非常量数据。
不能返回静态（全局）非常量数据的地址。
只能处理由调用者提供的数据。
不能依赖于单实例模式资源的锁。
不能调用不可重入的函数。

与线程安全的关系
可重入与线程安全两个概念都关系到函数处理资源的方式。但是，他们有一定的区别。可重入概念会影响函数的外部接口，而线程安全只关心函数的实现。

大多数情况下，要将不可重入函数改为可重入的，需要修改函数接口，使得所有的数据都通过函数的调用者提供。
要将非线程安全的函数改为线程安全的，则只需要修改函数的实现部分。一般通过加入同步机制以保护共享的资源，使之不会被几个进程同时访问。
因此，相对线程安全来说，可重入性是更基本的特性，它可以保证线程安全：即，所有的可重入函数都是线程安全的，但并非所有的线程安全函数都是可重入的。

2、
调用一个函数（假设该函数是正确的）操作某对象常常会使该对象暂时陷入不可用的状态（通常称为不稳定状态），等到操作完全结束，该对象才会重新回到完全可用的状态。如果其他线程企图访问一个处于不可用状态的对象，该对象将不能正确响应从而产生无法预料的结果，如何避免这种情况发生是线程安全性的核心问题。

一般准则
　　如果一个函数在其文档中没有特别注明具备线程安全性，则应该认为它不具备。许多库大量使用了内部的静态数据，除非它是为多线程应用所设计，否则要牢记其内部数据可能没有利用互斥量进行适当的保护。类似，如果类的成员函数在其文档中没有特别注明对于多线程应用是安全的话，则认为它不安全。两个线程去操作相同的对象会引起问题，这是显而易见的，然而，即使两个线程去操作不同的物体依然会引起问题。出于多种原因，许多类使用了内部静态数据或者在多个看上去明显不同的对象间共享实现细则，
　　以下给出几个一般准则：
　　操作系统提供的API具备线程安全性
　　POSIX线程标准要求C标准库中的大多数函数具备线程安全性，少数例外会在C标准中注明。
　　对于Windows提供的C标准库，如果所使用的版本没有问题，而且进行了正确的初始化，他们都是安全的。
　　C++标准库的线程安全性不是很明确，它在很大程度上依赖于使用的编译器。标准模板库线程安全性的SGI准则作为实际中的标准取得很大进展，但并不是统一的标准。

They are categorized in three groups: Creational, Structural, and Behavioral.

Abstract Factory
definition
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Factory Method
definition
Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Builder
definition
Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Prototype
definition
Specify the kind of objects to create using a prototypical instance, and create new objects by copying this prototype.

Singleton
definition
Ensure a class has only one instance and provide a global point of access to it.
 

Adapter
definition
Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
 
Bridge
definition
Decouple an abstraction from its implementation so that the two can vary independently.

Composite
definition
Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
 
Decorator
definition
Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
 
Facade
definition
Provide a unified interface to a set of interfaces in a subsystem. Fa鏰de defines a higher-level interface that makes the subsystem easier to use.

Flyweight
definition
Use sharing to support large numbers of fine-grained objects efficiently.
 
Proxy
definition
Provide a surrogate or placeholder for another object to control access to it. 

Chain of Responsibility
definition
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
 
Command
definition
Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
 
Interpreter
definition
Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. 

Iterator
definition
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. 

Mediator
definition
Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
 
Memento
definition
Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
 
Observer
definition
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
 
State
definition
Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
 
Strategy
definition
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. 

Template
definition
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.

 
Visitor
definition
Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

2.用命建数据库和表时在最后都加上default charset=gbk;

如：create database xx default charset=gbk;

CREATE TABLE tt (
 ...
) default charset=gbk;

3.在连接语句中加入：

mysqlUrl = "jdbc:mysql://localhost:3306/ebook?useUnicode=true&characterEncoding=gb2312";

4.应注意在jsp页面中中入：<%@ page language="java" contentType="text/html; charset=GB2312" pageEncoding="GB2312"%>

5.在jsp中获得字段时，在往后如数据库表中插入记录时应注意将sql处理。如下：

String sql = "xx";

System.out.println(sql);  //显示没有处理sql

sql = new String(sql.getBytes("ISO8859-1"), "GB2312");  //用于将汉字处理，不然的话你会发现数据库表中存的不是汉字

System.out.println(sql);

int i = stmt.executeUpdate(sql); // 执行sql语句：

6.网上有人说JDBC驱动也有可能出现汉字乱码，最好使用较高版本的Mysql驱动