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Spring对静态变量无法注入
问题
今天在学习的过程中想写一个连接和线程绑定的JDBCUtils工具类，但测试时发现一直报空指针异常，上网查了之后Spring并不支持对静态成员变量注入，所以光试用@Autowired肯定是不行的。
可是我们编写工具类时肯定是要使用静态变量和方法的，我总结一下我用过可以实现对静态成员变量注入的方法。
@Componentpublic class JDBCUtils { @Autowired private static ComboPooledDataSource dataSource; private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>(); public static Connection getThreadConnection(){ Connection conn = tl.get(); if (conn == null){ conn = getConnection(); tl.set(conn); } return conn; } public static DataSource getDataSource(){ return dataSource; } public static Connection getConnection(){ Connection connection = null; try { connection = dataSource.getConnection(); } catch (SQLException e) { e.printStackTrace(); } return connection; } public static void removeThreadConnection(){ tl.remove(); }}set方法注入
注解方式
在类前加@Component注解，在set方法上加 @Autowired注解，这里注意两点
配置文件里已经配置了变量的相关参数
静态变量自动生成set方法时会有static修饰，要去掉，否则还是无法注入
@Componentpublic class JDBCUtils { private static ComboPooledDataSource dataSource; @Autowired public void setDataSource(ComboPooledDataSource dataSource) { JDBCUtils.dataSource = dataSource; }xml方式
同样注意将set方法上的static去掉
public class JDBCUtils { private static ComboPooledDataSource dataSource; public void setDataSource(ComboPooledDataSource dataSource) { this.dataSource = dataSource; } private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>(); public static Connection getThreadConnection(){ Connection conn = tl.get(); if (conn == null){ conn = getConnection(); tl.set(conn); } return conn; } public static DataSource getDataSource(){ return dataSource; } public static Connection getConnection(){ Connection connection = null; try { connection = dataSource.getConnection(); } catch (SQLException e) { e.printStackTrace(); } return connection; } public static void removeThreadConnection(){ tl.remove(); }} <bean id="JDBCUtils" class="com.cc.utils.JDBCUtils"> <property name="dataSource" ref="dataSource"></property> </bean>@PostConstruct注解方式注入
用@PostConstruct加在init方法上，在类初始化后执行该方法，对成员变量赋值。在这之前，我们要改造一下工具类，去掉我们想注入变量的static的修饰符，这样我们就可以用@Autowired实现对其注入。
然后加一个静态的类自身的引用对象，当我们想要变量时通过这个引用对象来获取。
@Componentpublic class JDBCUtils { @Autowired private ComboPooledDataSource dataSource; private static JDBCUtils jdbcUtils; @PostConstruct public void init(){ jdbcUtils = this; this.dataSource = dataSource; } private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>(); public static Connection getThreadConnection(){ Connection conn = tl.get(); if (conn == null){ conn = getConnection(); tl.set(conn); } return conn; } public static DataSource getDataSource(){ return jdbcUtils.dataSource; } public static Connection getConnection(){ Connection connection = null; try { connection = jdbcUtils.dataSource.getConnection(); } catch (SQLException e) { e.printStackTrace(); } return connection; } public static void removeThreadConnection(){ tl.remove(); }}当然这种用初始化方法也可以用xml配置，原理一样。
public class JDBCUtils { private ComboPooledDataSource dataSource; public void setDataSource(ComboPooledDataSource dataSource) { this.dataSource = dataSource; } private static JDBCUtils jdbcUtils; public void init(){ jdbcUtils = this; this.dataSource = dataSource; } private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>(); public static Connection getThreadConnection(){ Connection conn = tl.get(); if (conn == null){ conn = getConnection(); tl.set(conn); } return conn; } public static DataSource getDataSource(){ return jdbcUtils.dataSource; } public static Connection getConnection(){ Connection connection = null; try { connection = jdbcUtils.dataSource.getConnection(); } catch (SQLException e) { e.printStackTrace(); } return connection; } public static void removeThreadConnection(){ tl.remove(); }}<bean id="JDBCUtils" class="com.cc.utils.JDBCUtils" init-method="init"> <property name="dataSource" ref="dataSource"></property> </bean>静态方法注入bean失败原因
今天在写redission 的一个工具类的时候,随手写出下面的代码
package com.wt.redission.wtredission.utils; import org.redisson.api.*;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Component; import javax.annotation.PostConstruct; @Componentpublic class RedissionUtilserror { @Autowired private static RedissonClient redissonClient; public static RLock getRLock(String objectName) { RLock rLock =redissonClient.getLock(objectName); return rLock; } //根据名字获取map public static <K, V> RMap<K, V> getRMap(String objectName) { RMap<K, V> map = redissonClient.getMap(objectName); return map; } //根据名字和值设置map public static void setMap(String objectName,Object key,Object value){ RMap<Object, Object> map =redissonClient.getMap(objectName); map.put(key,value); } //根据名字获取set public static <V> RSet<V> getSet(String objectName) { RSet<V> set = redissonClient.getSet(objectName); return set; } //根据名字和值设置set public static void setSet(String objectName,Object value){ RSet<Object> set = redissonClient.getSet(objectName); set.add(value); } //根据名字获取list public static <V> RList<V> getRList(String objectName) { RList<V> rList = redissonClient.getList(objectName); return rList; } //根据名字和值设置list public static void setList(String objectName, int index,Object element ){ RList<Object> objectRList = redissonClient.getList(objectName); objectRList.set(index,element); } //根据名字获取bucket public static <T> RBucket<T> getRBucket(String objectName) { RBucket<T> bucket = redissonClient.getBucket(objectName); return bucket; } //根据名字和值 设置对应的bucket public static <T> T setBucket(String objectName,String value){ RBucket<Object> bucket = redissonClient.getBucket(objectName); bucket.set(value); T t= (T) bucket.get(); //值类型由返回值确定 return t; } }乍一看好像没问题 我写一个静态方法 然后在方法中使用静态变量redissonClient ,哇....,一切看得如此正常
当我开始测试时,NPE.............,我去这是怎么回事,自己在想这不科学啊,怎么会空指针,于是我开始找原因
最后发现是基础不牢啊............，对jvm的类加载机制几乎就没考虑,简要说要错误的原因
jvm在进行类加载的时候,首先会加载类变量,类方法,也就是我这里被static修饰的方法,然后当我调用静态方法进行使用的时候,会使用到redissionClient,注意这个redissionClient是通过autowired进来的,关键问题就在这里,autowired的底层是通过构造器和set方法注入bean的
redissionClient被static修饰 并且还是一个接口 在被调用的时候肯定没有实例化
下面提供三种方式正确使用
方式一
package com.wt.redission.wtredission.utils; import org.redisson.api.*;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Component; import java.util.List; @Componentpublic class RedissionUtils { private static RedissonClient redissonClient; @Autowired public RedissionUtils(RedissonClient redissonClient){ RedissionUtils.redissonClient=redissonClient; } public static RLock getRLock(String objectName) { RLock rLock = redissonClient.getLock(objectName); return rLock; } //根据名字获取map public static <K, V> RMap<K, V> getRMap(String objectName) { RMap<K, V> map = redissonClient.getMap(objectName); return map; } //根据名字和值设置map public static void setMap(String objectName,Object key,Object value){ RMap<Object, Object> map =redissonClient.getMap(objectName); map.put(key,value); } //根据名字获取set public static <V> RSet<V> getSet(String objectName) { RSet<V> set = redissonClient.getSet(objectName); return set; } //根据名字和值设置set public static void setSet(String objectName,Object value){ RSet<Object> set = redissonClient.getSet(objectName); set.add(value); } //根据名字获取list public static <V> RList<V> getRList(String objectName) { RList<V> rList = redissonClient.getList(objectName); return rList; } //根据名字和值设置list public static void setList(String objectName, int index,Object element ){ RList<Object> objectRList = redissonClient.getList(objectName); objectRList.set(index,element); } //根据名字获取bucket public static <T> RBucket<T> getRBucket(String objectName) { RBucket<T> bucket = redissonClient.getBucket(objectName); return bucket; } //根据名字和值 设置对应的bucket public static <T> T setBucket(String objectName,String value){ RBucket<Object> bucket = redissonClient.getBucket(objectName); bucket.set(value); T t= (T) bucket.get(); //值类型由返回值确定 return t; } }方式二
package com.wt.redission.wtredission.utils; import org.redisson.api.*;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Component; import javax.annotation.PostConstruct; @Componentpublic class RedissionUtils2 { @Autowired RedissonClient redissonClient; public static RedissionUtils2 redissionUtils; @PostConstruct public void init(){ redissionUtils=this; redissionUtils.redissonClient=this.redissonClient; } public static RLock getRLock(String objectName) { RLock rLock = redissionUtils.redissonClient.getLock(objectName); return rLock; } //根据名字获取map public static <K, V> RMap<K, V> getRMap(String objectName) { RMap<K, V> map = redissionUtils.redissonClient.getMap(objectName); return map; } //根据名字和值设置map public static void setMap(String objectName,Object key,Object value){ RMap<Object, Object> map =redissionUtils.redissonClient.getMap(objectName); map.put(key,value); } //根据名字获取set public static <V> RSet<V> getSet(String objectName) { RSet<V> set = redissionUtils.redissonClient.getSet(objectName); return set; } //根据名字和值设置set public static void setSet(String objectName,Object value){ RSet<Object> set = redissionUtils.redissonClient.getSet(objectName); set.add(value); } //根据名字获取list public static <V> RList<V> getRList(String objectName) { RList<V> rList = redissionUtils.redissonClient.getList(objectName); return rList; } //根据名字和值设置list public static void setList(String objectName, int index,Object element ){ RList<Object> objectRList = redissionUtils.redissonClient.getList(objectName); objectRList.set(index,element); } //根据名字获取bucket public static <T> RBucket<T> getRBucket(String objectName) { RBucket<T> bucket = redissionUtils.redissonClient.getBucket(objectName); return bucket; } //根据名字和值 设置对应的bucket public static <T> T setBucket(String objectName,String value){ RBucket<Object> bucket = redissionUtils.redissonClient.getBucket(objectName); bucket.set(value); T t= (T) bucket.get(); //值类型由返回值确定 return t; } }方式三 通过spring上下文获取
package com.wt.redission.wtredission.utils; import io.micrometer.core.instrument.util.StringUtils;import org.springframework.beans.BeansException;import org.springframework.beans.factory.NoSuchBeanDefinitionException;import org.springframework.context.ApplicationContext;import org.springframework.context.ApplicationContextAware;import org.springframework.context.annotation.Scope;import org.springframework.stereotype.Component; import javax.servlet.http.HttpServletRequest; @Component@Scope("singleton")public class SpringUtil implements ApplicationContextAware { private static ApplicationContext applicationContext; @Override public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {SpringUtil.applicationContext = applicationContext; } public static ApplicationContext getApplicationContext() { return applicationContext; } @SuppressWarnings("unchecked") public static <T> T getBean(String name) throws BeansException { return (T) applicationContext.getBean(name); } public static <T> T getBean(Class<T> clz) throws BeansException { return (T)applicationContext.getBean(clz); } public static boolean containsBean(String name) { return applicationContext.containsBean(name); } public static boolean isSingleton(String name) throws NoSuchBeanDefinitionException { return applicationContext.isSingleton(name); } public static Class<?> getType(String name) throws NoSuchBeanDefinitionException { return applicationContext.getType(name); } public static String[] getAliases(String name) throws NoSuchBeanDefinitionException { return applicationContext.getAliases(name); } public static String getJwtToken(HttpServletRequest servletRequest, String tokenId) {String token = servletRequest.getHeader(tokenId);if (StringUtils.isBlank(token)) {token = servletRequest.getParameter(tokenId);}return token;}}感谢各位的阅读，以上就是“如何解决Spring对静态变量无法注入的问题”的内容了，经过本文的学习后，相信大家对如何解决Spring对静态变量无法注入的问题这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是编程网，小编将为大家推送更多相关知识点的文章，欢迎关注！
