Spring對(duì)靜態(tài)變量無(wú)法注入的解決方案

更新時(shí)間：2021年07月09日 09:44:51 作者：qq_42524262

這篇文章主要介紹了使用Spring對(duì)靜態(tài)變量無(wú)法注入的解決方案，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。

Spring對(duì)靜態(tài)變量無(wú)法注入

問(wèn)題

今天在學(xué)習(xí)的過(guò)程中想寫一個(gè)連接和線程綁定的JDBCUtils工具類，但測(cè)試時(shí)發(fā)現(xiàn)一直報(bào)空指針異常，上網(wǎng)查了之后Spring并不支持對(duì)靜態(tài)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(shí)肯定是要使用靜態(tài)變量和方法的，我總結(jié)一下我用過(guò)可以實(shí)現(xiàn)對(duì)靜態(tài)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();
    }
}

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(diǎn)

1.配置文件里已經(jīng)配置了變量的相關(guān)參數(shù)

2.靜態(tài)變量自動(dòng)生成set方法時(shí)會(huì)有static修飾，要去掉，否則還是無(wú)法注入

@Component
public 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 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，在類初始化后執(zhí)行該方法，對(duì)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(shí)現(xiàn)對(duì)其注入。

然后加一個(gè)靜態(tài)的類自身的引用對(duì)象，當(dāng)我們想要變量時(shí)通過(guò)這個(gè)引用對(duì)象來(lái)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(dāng)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>

靜態(tài)方法注入bean失敗原因

今天在寫redission 的一個(gè)工具類的時(shí)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(jù)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(jù)名字和值設(shè)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(jù)名字獲取set
    public static <V> RSet<V> getSet(String objectName) {
        RSet<V> set = redissonClient.getSet(objectName);
        return set;
    }
 
    //根據(jù)名字和值設(shè)置set
    public static void setSet(String objectName,Object value){
        RSet<Object> set = redissonClient.getSet(objectName);
        set.add(value);
    }
 
    //根據(jù)名字獲取list
    public static  <V> RList<V> getRList(String objectName) {
        RList<V> rList = redissonClient.getList(objectName);
        return rList;
    } 
 
    //根據(jù)名字和值設(shè)置list
    public static void setList(String objectName, int  index,Object element ){
        RList<Object> objectRList = redissonClient.getList(objectName);
        objectRList.set(index,element);
    }
 
    //根據(jù)名字獲取bucket
    public static <T> RBucket<T> getRBucket(String objectName) {
        RBucket<T> bucket = redissonClient.getBucket(objectName);
        return bucket;
    }
 
    //根據(jù)名字和值 設(shè)置對(duì)應(yīng)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(méi)問(wèn)題我寫一個(gè)靜態(tài)方法然后在方法中使用靜態(tài)變量redissonClient ,哇....,一切看得如此正常

當(dāng)我開始測(cè)試時(shí),NPE.............,我去這是怎么回事,自己在想這不科學(xué)啊,怎么會(huì)空指針,于是我開始找原因

最后發(fā)現(xiàn)是基礎(chǔ)不牢啊............，對(duì)jvm的類加載機(jī)制幾乎就沒(méi)考慮,簡(jiǎn)要說(shuō)要錯(cuò)誤的原因

jvm在進(jìn)行類加載的時(shí)候,首先會(huì)加載類變量,類方法,也就是我這里被static修飾的方法,然后當(dāng)我調(diào)用靜態(tài)方法進(jìn)行使用的時(shí)候,會(huì)使用到redissionClient,注意這個(gè)redissionClient是通過(guò)autowired進(jìn)來(lái)的,關(guān)鍵問(wèn)題就在這里,autowired的底層是通過(guò)構(gòu)造器和set方法注入bean的

redissionClient被static修飾并且還是一個(gè)接口在被調(diào)用的時(shí)候肯定沒(méi)有實(shí)例化

下面提供三種方式正確使用

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; 
@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(jù)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(jù)名字和值設(shè)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(jù)名字獲取set
    public static <V> RSet<V> getSet(String objectName) {
        RSet<V> set = redissonClient.getSet(objectName);
        return set;
    }
 
    //根據(jù)名字和值設(shè)置set
    public static void setSet(String objectName,Object value){
        RSet<Object> set = redissonClient.getSet(objectName);
        set.add(value);
    }
 
    //根據(jù)名字獲取list
    public static  <V> RList<V> getRList(String objectName) {
        RList<V> rList = redissonClient.getList(objectName);
        return rList;
    } 
 
    //根據(jù)名字和值設(shè)置list
    public static void setList(String objectName, int  index,Object element ){
        RList<Object> objectRList = redissonClient.getList(objectName);
        objectRList.set(index,element);
    }
 
    //根據(jù)名字獲取bucket
    public static <T> RBucket<T> getRBucket(String objectName) {
        RBucket<T> bucket = redissonClient.getBucket(objectName);
        return bucket;
    }
 
    //根據(jù)名字和值 設(shè)置對(duì)應(yīng)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; 
@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(jù)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(jù)名字和值設(shè)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(jù)名字獲取set
    public static <V> RSet<V> getSet(String objectName) {
        RSet<V> set = redissionUtils.redissonClient.getSet(objectName);
        return set;
    }
 
    //根據(jù)名字和值設(shè)置set
    public static void setSet(String objectName,Object value){
        RSet<Object> set = redissionUtils.redissonClient.getSet(objectName);
        set.add(value);
    }
 
    //根據(jù)名字獲取list
    public static  <V> RList<V> getRList(String objectName) {
        RList<V> rList = redissionUtils.redissonClient.getList(objectName);
        return rList;
    } 
 
    //根據(jù)名字和值設(shè)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(jù)名字獲取bucket
    public static <T> RBucket<T> getRBucket(String objectName) {
        RBucket<T> bucket = redissionUtils.redissonClient.getBucket(objectName);
        return bucket;
    }
 
    //根據(jù)名字和值 設(shè)置對(duì)應(yīng)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(guò)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(yīng)用上下文環(huán)境.
     */
    private static ApplicationContext applicationContext;
 
    /**
     * 實(shí)現(xiàn)ApplicationContextAware接口的回調(diào)方法，設(shè)置上下文環(huán)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(duì)象.
     *
     * <br>🌹param: name
     * <br>🌹return: Object 一個(gè)以所給名字注冊(cè)的bean的實(shí)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(duì)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(gè)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(cè)的bean定義是一個(gè)singleton還是一個(gè)prototype。
     * 如果與給定名字相應(yīng)的bean定義沒(méi)有被找到，將會(huì)拋出一個(gè)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(cè)對(duì)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(qǐng)求頭獲取請(qǐng)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;
	}
}

以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持腳本之家。

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