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這篇文章主要講解了“如何解決Spring對靜態變量無法注入的問題”，文中的講解內容簡單清晰，易于學習與理解，下面請大家跟著小編的思路慢慢深入，一起來研究和學習“如何解決Spring對靜態變量無法注入的問題”吧！
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。
@Component
public 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();
}
}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
@Component
public class JDBCUtils {
private static ComboPooledDataSource dataSource;
@Autowired
public void setDataSource(ComboPooledDataSource dataSource) {
JDBCUtils.dataSource = dataSource;
}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 id="JDBCUtils" class="com.cc.utils.JDBCUtils"> <property name="dataSource" ref="dataSource"></property> </bean>
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。
@Component
public 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 id="JDBCUtils" class="com.cc.utils.JDBCUtils" init-method="init"> <property name="dataSource" ref="dataSource"></property> </bean>
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;
@Component
public 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;
@Component
public 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;
}
}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;
@Component
public 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;
}
}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;
}
}感謝各位的閱讀，以上就是“如何解決Spring對靜態變量無法注入的問題”的內容了，經過本文的學習后，相信大家對如何解決Spring對靜態變量無法注入的問題這一問題有了更深刻的體會，具體使用情況還需要大家實踐驗證。這里是億速云，小編將為大家推送更多相關知識點的文章，歡迎關注！
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