属性在基于Spring Boot的Java应用程序中不起作用

Question

i have spring boot based java app. i am using java.util.properties to read the properties from application.properties file present in src/main/resources (default path). i have just defined the getters and setters to read the props. following is the code :

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;
    }

}

now, in another class, just creating the object of this class and trying to call the getHost() method to get the host ip address.

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

getting following exception :

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

without using properties class, if i just hard code the IP address, it absolutely works fine. dont know what the issue in the code so the property reader is not working.

following the application.properties content:

spring.mysql.host=35.154.83.162

Update ::::

here is the code updated :

@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");

}
}

getting NPE exception :

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

Answer 1

In spring boot you don't need to read properties manually (specially application.properties ).
application.properties (.yml|.yaml) is by default loaded into spring Environemnt class.

@Component
public class PropertyReader {

    @Autowired
    private Environment env;

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

To use it, just autowire PropertyReader and call getHost() method.
Even you don't need to write this class, you can directly use Environment class as well.

EDIT (After Question Update)

Solution 1 (Using Environment)

Move your constructor code to init method, at that point env is not initialized.

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 
    }
}

Solution 2 (Using @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
}