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Spring对静态变量无法注入的解决方案

时间：2021-09-30 01:26     来源/作者：42524262

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。

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@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

在类前加@Component注解，在set方法上加 @Autowired注解，这里注意两点

1.配置文件里已经配置了变量的相关参数

2.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

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@Component
public 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

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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();
    }
}
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<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。

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@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();
    }
}

当然这种用初始化方法也可以用xml配置，原理一样。

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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();
    }
}
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<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

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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

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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

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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

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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工具类.
 *
 * @author:Hohn
 */
@Component
@Scope("singleton")
public class SpringUtil implements ApplicationContextAware {
 
    /**
     * Spring应用上下文环境.
     */
    private static ApplicationContext applicationContext;
 
    /**
     * 实现ApplicationContextAware接口的回调方法，设置上下文环境
     *
     * <br>
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spring 静态变量 无法注入 

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