public class Foo {
    private SomeObject someName;
    // do stuff
    public void someMethod() {
        if (this.someName != null) {
            // do some stuff
        }
    }
}

变量的初始值
在程序里的每一个变量在使用之前都必须有一个值:
每个类变量,实例变量或者数据部分在被创建的时间会被初始化为一个默认值.
[...] 对于所有引用类型,默认值为null.

public class SessionRepositoryRequestWrapper extends HttpServletRequestWrapper {
 
    public SessionRepositoryRequestWrapper(HttpServletRequest original) {
        super(original);
    }
 
    public HttpSession getSession() {
        return getSession(true);
    }
 
    public HttpSession getSession(boolean createNew) {
        // create an HttpSession implementation from Spring Session
    }
 
    // ... other methods delegate to the original HttpServletRequest ...
}

public class SessionRepositoryFilter implements Filter {
 
    public doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        SessionRepositoryRequestWrapper customRequest =
              new SessionRepositoryRequestWrapper(httpRequest);
 
        chain.doFilter(customRequest, response, chain);
    }
 
    // ...
}

<dependency>
  <groupId>org.springframework.session</groupId>
  <artifactId>spring-session-data-redis</artifactId>
  <version>1.0.0.BUILD-SNAPSHOT</version>
  <type>pom<type>
</dependency>
<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-web</artifactId>
  <version>4.1.3.RELEASE</version>
</dependency>

<filter>
    <filter-name>spring-session</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
    <init-param>
        <param-name>targetBeanName</param-name>
        <param-value>springSession</param-value>
    </init-param>
</filter>
 
<filter-mapping>
    <filter-name>spring-session</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

<!--这里添加的是Redis，因为使用的是Spring里自带的Redis的Session策略 -->
<bean id="v2redisConnectionFactory" class="org.springframework.data.redis.connection.jedis.JedisConnectionFactory"
    p:host-name="10.0.0.40" p:port="6379" p:use-pool="true" p:database="8" />
 
<bean id="stringRedisSerializer"
    class="org.springframework.data.redis.serializer.StringRedisSerializer"/>
 
<bean id="v2redisTemplate" class="org.springframework.data.redis.core.RedisTemplate"
    p:connection-factory-ref="v2redisConnectionFactory"
    p:keySerializer-ref="stringRedisSerializer"
    p:valueSerializer-ref="stringRedisSerializer"
    p:hashKeySerializer-ref="stringRedisSerializer"
    p:hashValueSerializer-ref="stringRedisSerializer" />
<!-- 这里的是为了下面的 Session策略过滤器提供构造函数传入的参数，因为Session过滤器要依赖该对象来构造，所以创建一个先 -->
<bean name="redisOperationsSessionRepository" class="org.springframework.session.data.redis.RedisOperationsSessionRepository">
    <constructor-arg ref="v2redisConnectionFactory"></constructor-arg>
</bean>
 
<!-- 这个是Session策略过滤器，即将容器原有的Session持久化机制，代替为Spring的 Redis持久化Session机制。 -->
<!-- 注意，这个名字与 web.xml里的targetBean的下value是要一致的。 -->
<bean name="springSession" class="org.springframework.session.web.http.SessionRepositoryFilter">
    <constructor-arg ref="redisOperationsSessionRepository"></constructor-arg>
</bean>

1.Tomcat配置：
     <Connector port="8080" protocol="HTTP/1.1"
               connectionTimeout="20000"
               redirectPort="8443"  URIEncoding="UTF-8"  useBodyEncodingForURI="true"/>
2.文件：必须设置统一编码
     JSP：<%@ page language="java" pageEncoding="UTF-8"%>
     HTML：<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
     ServletRequest:
     request.setCharacterEncoding("UTF-8")是把提交内容的字符集设为UTF－8
     ServletResponse:
     response.setCharacterEncoding("UTF-8")
3. MySQL
     default-character-set=utf-8
     mysql里数据库和表也都设为utf8_unicode_ci
     jdbc:mysql://localhost/mydb?useUnicode=true&characterEncoding=utf-8
4. PostgreSQL
     postgresql.conf文件中的client_encoding改成UTF-8,
5. 注意：使用 FileInputStream 读取文件时，使用的是系统默认的字符编码。如果用在UTF-8系统中，很容易出现问题。
     这时，可以使用：
     FileInputStream  in  =  new  FileInputStream("g:\\yufa.txt");
     InputStreamReader reader = new InputStreamReader (in,"GBK"); // 用这个方法读取，并指定编码
     
6. Java中的 properties 文件，如果是使用 utf-8 文件格式的，这个格式是无 BOM 格式的UTF-8的。切记
7. Spring
 
<filter>
    <filter-name>CharacterEncodingFilter</filter-name>
    <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
    <init-param>
        <param-name>encoding</param-name>
        <param-value>utf-8</param-value>
    </init-param>
    <init-param>
        <param-name>forceEncoding</param-name>
        <param-value>true</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>CharacterEncodingFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

response.setContentType指定 HTTP 响应的编码,同时指定了浏览器显示的编码.
response.setCharacterEncoding设置HTTP 响应的编码,如果之前使用response.setContentType设置了编码格式,则使用response.setCharacterEncoding指定的编码格式覆盖之前的设置.与response.setContentType相同的是,调用此方法,必须在getWriter执行之前或者response被提交之前.

      Object
     /      \
    /        \
String     Integer

     Object
     /
    /
   A
  /
/
B