mysql属性在基于SpringBoot的java应用程序中不起作用

我有基于spring-boot的java应用程序。我使用java.util.properties从src/main/resources（默认路径）中的application.properties文件读取属性。我刚刚定义了读取道具的getter和setter。代码如下：

public class PropertyReader {

    String host;

    public String getHost() {

        Properties properties = new Properties();
        try {
            File file = ResourceUtils.getFile("classpath:application.properties");
            InputStream in = new FileInputStream(file);
            properties.load(in);
        } catch (IOException e) {

        }
        return host = properties.getProperty("spring.mysql.host");
    }

    public void setHost(String host) {
        this.host = host;
    }

}

现在，在另一个类中，只需创建这个类的对象并尝试调用gethost（）方法来获取主机ip地址。

PropertyReader pr = new PropertyReader();
String host = pr.getHost();
PoolProperties p = new PoolProperties();
p.setUrl("jdbc:mysql://" + host + "/ci");

获取以下异常：

Caused by: java.net.UnknownHostException: null
        at java.net.InetAddress.getAllByName0(InetAddress.java:1280) ~[na:1.8.0_151]
        at java.net.InetAddress.getAllByName(InetAddress.java:1192) ~[na:1.8.0_151]
        at java.net.InetAddress.getAllByName(InetAddress.java:1126) ~[na:1.8.0_151]
        at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:188) ~[mysql-connector-java-5.1.45.jar!/:5.1.45]
        at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:300) ~[mysql-connector-java-5.1.45.jar!/:5.1.45]
        ... 44 common frames omitted

不使用properties类，如果我只是硬编码ip地址，它绝对可以正常工作。不知道代码中的问题是什么，因此属性读取器无法工作。
遵循application.properties内容：

spring.mysql.host=35.154.83.162

更新：：：：
以下是更新的代码：

@Component
@Configuration
public class UnitDBHelper {
@Autowired
    private Environment env;

public UnitDBHelper() {

        String host = env.getProperty("spring.mysql.host");     
        PoolProperties p = new PoolProperties();
        InputStream input = null;
        p.setUrl("jdbc:mysql://" + host + "/ci");

}
}

获取npe异常：

Error starting ApplicationContext. To display the auto-configuration report re-run your application with 'debug' enabled.
2018-05-30 15:42:29.532 ERROR 9870 --- [           main] o.s.boot.SpringApplication               : Application startup failed

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'unitDBHelper' defined in URL [jar:file:/tmp/unitdbamqpservice-0.0.1-SNAPSHOT.jar!/BOOT-INF/classes!/com/infy/ci/unitdbamqpservice/UnitDBHelper.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.infy.ci.unitdbamqpservice.UnitDBHelper$$EnhancerBySpringCGLIB$$24a2dca6]: Constructor threw exception; nested exception is java.lang.NullPointerException

Caused by: java.lang.NullPointerException: null
        at com.infy.ci.unitdbamqpservice.UnitDBHelper.<init>(UnitDBHelper.java:39) ~[classes!/:0.0.1-SNAPSHOT]
        at com.infy.ci.unitdbamqpservice.UnitDBHelper$$EnhancerBySpringCGLIB$$24a2dca6.<init>(<generated>) ~[classes!/:0.0.1-SNAPSHOT]
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_151]
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_151]
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_151]
        at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_151]
        at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:142) ~[spring-beans-4.3.14.RELEASE.jar!/:4.3.14.RELEASE]
        ... 27 common frames omitted

在springboot中，您不需要手动读取属性（特别是 application.properties ).
application.properties（.yml |.yaml）默认加载到spring环境类中。

@Component
public class PropertyReader {

    @Autowired
    private Environment env;

    public String getHost() {
        return env.getProperty("spring.mysql.host");
    }
}

要使用它，只需自动连线 PropertyReader 打电话给我 getHost() 方法。
即使不需要编写这个类，也可以直接使用 Environment 上课也是。
编辑（问题更新后）
解决方案1（使用环境）
此时，将构造函数代码移到init方法 env 未初始化。

public class UnitDBHelper implements InitializingBean {

    // your autowires

    @Override
    public void afterPropertiesSet() throws Exception {
        // your constructor code,
        // this will be called after injecting all beans
        // use `env` here 
    }
}

解决方案2（使用@value）

@Component
public class UnitDBHelper {

    @Value("${spring.mysql.host}")
    private String host;

    // you can still not use host in constructor
    // as it will be uninitialized 

    //  rest of code
}

mysql属性在基于SpringBoot的java应用程序中不起作用

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