解決Spring對靜態變量無法注入問題(轉)

問題
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並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();
    }
}

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

@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>

參考鏈接：https://blog.csdn.net/chen1403876161/article/details/53644024

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原文鏈接：https://blog.csdn.net/qq_42524262/article/details/97898796