今天在學習的過程中想寫一個連接和線程綁定的JDBCUtils工具類，但測試時發現一直報空指針異常，上網查了之后Spring并不支持對靜態成員變量注入，所以光試用@Autowired肯定是不行的。

可是我們編寫工具類時肯定是要使用靜態變量和方法的，我總結一下我用過可以實現對靜態成員變量注入的方法。

@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

在類前加@Component注解，在set方法上加 @Autowired注解，這里注意兩點

1.配置文件里已經配置了變量的相關參數

2.靜態變量自動生成set方法時會有static修飾，要去掉，否則還是無法注入

@Componentpublic class JDBCUtils { private static ComboPooledDataSource dataSource; @Autowired public void setDataSource(ComboPooledDataSource dataSource) {JDBCUtils.dataSource = dataSource; }xml方式

同樣注意將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 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，在類初始化后執行該方法，對成員變量賦值。在這之前，我們要改造一下工具類，去掉我們想注入變量的static的修飾符，這樣我們就可以用@Autowired實現對其注入。

然后加一個靜態的類自身的引用對象，當我們想要變量時通過這個引用對象來獲取。

@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(); }}

當然這種用初始化方法也可以用xml配置，原理一樣。

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 init-method='init'><property name='dataSource' ref='dataSource'></property> </bean>靜態方法注入bean失敗原因

今天在寫redission 的一個工具類的時候,隨手寫出下面的代碼

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; } //根據名字獲取map public static <K, V> RMap<K, V> getRMap(String objectName) {RMap<K, V> map = redissonClient.getMap(objectName);return map; } //根據名字和值設置map public static void setMap(String objectName,Object key,Object value){RMap<Object, Object> map =redissonClient.getMap(objectName);map.put(key,value); } //根據名字獲取set public static <V> RSet<V> getSet(String objectName) {RSet<V> set = redissonClient.getSet(objectName);return set; } //根據名字和值設置set public static void setSet(String objectName,Object value){RSet<Object> set = redissonClient.getSet(objectName);set.add(value); } //根據名字獲取list public static <V> RList<V> getRList(String objectName) {RList<V> rList = redissonClient.getList(objectName);return rList; } //根據名字和值設置list public static void setList(String objectName, int index,Object element ){RList<Object> objectRList = redissonClient.getList(objectName);objectRList.set(index,element); } //根據名字獲取bucket public static <T> RBucket<T> getRBucket(String objectName) {RBucket<T> bucket = redissonClient.getBucket(objectName);return bucket; } //根據名字和值 設置對應的bucket public static <T> T setBucket(String objectName,String value){RBucket<Object> bucket = redissonClient.getBucket(objectName);bucket.set(value);T t= (T) bucket.get(); //值類型由返回值確定return t; } }

乍一看好像沒問題 我寫一個靜態方法 然后在方法中使用靜態變量redissonClient ,哇....,一切看得如此正常

當我開始測試時,NPE.............,我去這是怎么回事,自己在想這不科學啊,怎么會空指針,于是我開始找原因

最后發現是基礎不牢啊............，對jvm的類加載機制幾乎就沒考慮,簡要說要錯誤的原因

jvm在進行類加載的時候,首先會加載類變量,類方法,也就是我這里被static修飾的方法,然后當我調用靜態方法進行使用的時候,會使用到redissionClient,注意這個redissionClient是通過autowired進來的,關鍵問題就在這里,autowired的底層是通過構造器和set方法注入bean的

redissionClient被static修飾 并且還是一個接口 在被調用的時候肯定沒有實例化

下面提供三種方式正確使用

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; } //根據名字獲取map public static <K, V> RMap<K, V> getRMap(String objectName) {RMap<K, V> map = redissonClient.getMap(objectName);return map; } //根據名字和值設置map public static void setMap(String objectName,Object key,Object value){RMap<Object, Object> map =redissonClient.getMap(objectName);map.put(key,value); } //根據名字獲取set public static <V> RSet<V> getSet(String objectName) {RSet<V> set = redissonClient.getSet(objectName);return set; } //根據名字和值設置set public static void setSet(String objectName,Object value){RSet<Object> set = redissonClient.getSet(objectName);set.add(value); } //根據名字獲取list public static <V> RList<V> getRList(String objectName) {RList<V> rList = redissonClient.getList(objectName);return rList; } //根據名字和值設置list public static void setList(String objectName, int index,Object element ){RList<Object> objectRList = redissonClient.getList(objectName);objectRList.set(index,element); } //根據名字獲取bucket public static <T> RBucket<T> getRBucket(String objectName) {RBucket<T> bucket = redissonClient.getBucket(objectName);return bucket; } //根據名字和值 設置對應的bucket public static <T> T setBucket(String objectName,String value){RBucket<Object> bucket = redissonClient.getBucket(objectName);bucket.set(value);T t= (T) bucket.get(); //值類型由返回值確定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; } //根據名字獲取map public static <K, V> RMap<K, V> getRMap(String objectName) {RMap<K, V> map = redissionUtils.redissonClient.getMap(objectName);return map; } //根據名字和值設置map public static void setMap(String objectName,Object key,Object value){RMap<Object, Object> map =redissionUtils.redissonClient.getMap(objectName);map.put(key,value); } //根據名字獲取set public static <V> RSet<V> getSet(String objectName) {RSet<V> set = redissionUtils.redissonClient.getSet(objectName);return set; } //根據名字和值設置set public static void setSet(String objectName,Object value){RSet<Object> set = redissionUtils.redissonClient.getSet(objectName);set.add(value); } //根據名字獲取list public static <V> RList<V> getRList(String objectName) {RList<V> rList = redissionUtils.redissonClient.getList(objectName);return rList; } //根據名字和值設置list public static void setList(String objectName, int index,Object element ){RList<Object> objectRList = redissionUtils.redissonClient.getList(objectName);objectRList.set(index,element); } //根據名字獲取bucket public static <T> RBucket<T> getRBucket(String objectName) {RBucket<T> bucket = redissionUtils.redissonClient.getBucket(objectName);return bucket; } //根據名字和值 設置對應的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(); //值類型由返回值確定return t; } }方式三 通過spring上下文獲取

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; /** * Spring Context工具類. * * @author:Hohn */@Component@Scope('singleton')public class SpringUtil implements ApplicationContextAware { /** * Spring應用上下文環境. */ private static ApplicationContext applicationContext; /** * 實現ApplicationContextAware接口的回調方法，設置上下文環境 * * <br>🌹param: applicationContext * @throws BeansException */ @Override public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {SpringUtil.applicationContext = applicationContext; } /** * 獲取ApplicationContext. * * <br>🌹return: ApplicationContext */ public static ApplicationContext getApplicationContext() {return applicationContext; } /** * 獲取對象. * * <br>🌹param: name * <br>🌹return: Object 一個以所給名字注冊的bean的實例 * @throws BeansException */ @SuppressWarnings('unchecked') public static <T> T getBean(String name) throws BeansException {return (T) applicationContext.getBean(name); } /** * 獲取類型為requiredType的對象. * * <br>🌹param: clz * <br>🌹return: * @throws BeansException */ public static <T> T getBean(Class<T> clz) throws BeansException {return (T)applicationContext.getBean(clz); } /** * 如果BeanFactory包含一個與所給名稱匹配的bean定義，則返回true * * <br>🌹param: name * <br>🌹return: boolean */ public static boolean containsBean(String name) {return applicationContext.containsBean(name); } /** * 判斷以給定名字注冊的bean定義是一個singleton還是一個prototype。 * 如果與給定名字相應的bean定義沒有被找到，將會拋出一個異常（NoSuchBeanDefinitionException） * <br>🌹param: name * <br>🌹return: boolean * @throws NoSuchBeanDefinitionException */ public static boolean isSingleton(String name) throws NoSuchBeanDefinitionException {return applicationContext.isSingleton(name); } /** * <br>🌹param: name * <br>🌹return: Class 注冊對象的類型 * @throws NoSuchBeanDefinitionException */ public static Class<?> getType(String name) throws NoSuchBeanDefinitionException {return applicationContext.getType(name); } /** * 如果給定的bean名字在bean定義中有別名，則返回這些別名 * * <br>🌹param: name * <br>🌹return: * @throws NoSuchBeanDefinitionException */ public static String[] getAliases(String name) throws NoSuchBeanDefinitionException {return applicationContext.getAliases(name); } /** * 請求頭獲取請求token * @param servletRequest * @return */public static String getJwtToken(HttpServletRequest servletRequest, String tokenId) {String token = servletRequest.getHeader(tokenId);if (StringUtils.isBlank(token)) {token = servletRequest.getParameter(tokenId);}return token;}}

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