如何使用 bean 构造函数在 Spring 中的 YAML 列表中创建 bean 列表？

Question

Based on the question:
dynamically generate list of beans from .yaml file in confg-server
I want to create the Server bean using his constructor and not the setter way.

I want a yaml file like:

test.servers:
    -
      name: test
      url: testurl
    -
      name: test2
      url: test2url

I want to have a class that contains all the servers like:

@ConfigurationProperties(prefix = "test")
@Component
public class RealStateApiClients {
    public List<Server> getServers() {
        return servers;
    }

    public void setServers(List<Server> servers) {
        this.servers = servers;
    }

    List<Server> servers = new LinkedList<>();
}

I want to have the class Server like this:

public class Server{
    private String name;
    private String url;

    public Server(String name, String url, String pass) {
        this.name = name;
        this.url = url;
    }
}

But when I launch the application I have the exception

Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.Server]: Is it an abstract class?; nested exception is java.lang.InstantiationException: com.Server
    at org.springframework.beans.BeanUtils.instantiate(BeanUtils.java:80) ~[spring-beans-4.3.2.RELEASE.jar:4.3.2.RELEASE]
    at org.springframework.beans.AbstractNestablePropertyAccessor.newValue(AbstractNestablePropertyAccessor.java:914) ~[spring-beans-4.3.2.RELEASE.jar:4.3.2.RELEASE]
    ... 33 common frames omitted
Caused by: java.lang.InstantiationException: com.Server
    at java.lang.Class.newInstance(Class.java:427) ~[na:1.8.0_101]
    at org.springframework.beans.BeanUtils.instantiate(BeanUtils.java:77) ~[spring-beans-4.3.2.RELEASE.jar:4.3.2.RELEASE]
    ... 34 common frames omitted
Caused by: java.lang.NoSuchMethodException: com.Server.<init>()
    at java.lang.Class.getConstructor0(Class.java:3082) ~[na:1.8.0_101]
    at java.lang.Class.newInstance(Class.java:412) ~[na:1.8.0_101]
    ... 35 common frames omitted

If I define Server like this, all works:

public class Server{

    private String name;
    private String url;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }
}

what I am doing wrong?
thanks!

Answer 1

OMG I made the rookie mistake of spring!

The Server class must be defined like:

public class Server{
    private String name;
    private String url;

    public Server(String name, String url, String pass) {
        this.name = name;
        this.url = url;
    }

    public Server(){}

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }
}

So with a default constructor.

Take consideration that the Server class is no initialised with the constructor method and if you want to apply some logic in the constructor this solution does not work, however you can make the Server have a

public void init(){
  //do something
}

method and the RealStateApiClients class have a

@PostConstruct
public void initAllServers() {
  for (Server server: servers) {
    server.init();
  }
}

And now we have a initialization logic for each Server

Answer 2

Unfortunately @ConfigurationProperties classes have to have:

• default constructor
• setters