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Parse a yaml with placeholder in Java

I have a yaml file which consist of placeholders to be taken from environment variable. I want to parse it to a custom class. Currently I am using snakeyaml library to parse it and its populating the bean correctly, how can I resolve environment variables using snakeyaml or any other library in Java.
datasource:
url: {url_environment_variable}
foo: bar
username: {user_name_environment_variable}
password: {password_environment_variable}
#Getter
#Setter
public class DataSource {
private String url;
private String foo;
private String username;
private String password;
}
Parsing code below
Constructor c = new Constructor(MyDataSource.class);
Yaml yaml = new Yaml(c);
MyDataSource myData = yaml.loadAs(inputStream, MyDataSource.class);
The problem is I am yet to find a way to resolve placeholders. People were able to solve it using python and is available in question -
How to replace environment variable value in yaml file to be parsed using python script
How can I do the same in Java. I can add a new dependency if required.
PS - It's not a Spring Boot Project so standard Spring placeholder replacements can not be used.

The easiest way would be to do it in two passes. First deserialize into MyDataSource as you’re doing already. Then use reflection to iterate over all fields of the instance, and if the value starts with a curly brace and ends with one, extract the key, and get the value from System.getenv map. See this answer for code.
If you want to do it in one pass, then you need to use the snakeyaml event-driver parser. For every key, you resolve the value as described above, and then based on the key name, set the corresponding field in the MyDataSource class, either using reflection, or simple if-else. For an example of the event-driven parser, see this class; it’s not Java, it’s Kotlin, but it may be the only JVM language example you’ll find.

Spring Boot YAML: set null property value

I'm having trouble setting null as a property value.
This is how the value is defined in YAML file:
my-property: null
This is how I inject it in code:
#Value("${my-property}")
private String myProperty;
For some reason, Spring keeps injecting an empty string ("") instead of null. Am I missing something or is this an error in Spring?

You can't, but this is not because YAML. YAML supports null as per:
Empty field in yaml
This is because of a Spring processor class, which turns "null" values into empty strings, see here:
https://github.com/spring-projects/spring-framework/issues/19986

Try to use instead of $ use this #.
Or try this #Value("${<my-property>}")

I had a similar use case, and had to go through quite a bit of trial and error.
The following two-step solution worked for me
Don't define my-property in the YAML file. This way Spring cannot (annoyingly) inject a stringified value like "" or "~" or "null", and will be forced to look for a default value.
Use #{null} as the default value. This is an SpEL expression that evaluates to null.
In your example, it will be
#Value("${my-property:#{null}}")
private String myProperty;
Be careful not to mess up with the curly brackets.

Injecting property in Springboot using #Value annotation

I am using Spring and Java for an application, and I want to use the #Value annotation to inject the value of the property, my use case is that I want first to check if that property exists as system property (so it takes priority), and otherwise default to a configuration property (existing in the properties file)
In the commented code you can see what I am trying to achieve, is that possible to default to something else that a simple string? If it is not, how can I achieve this?
//#Value("#{configProperties['local.datasource.username']}") THIS IS THE ORIGINAL CODE
//#Value("#{systemProperties['some.key'] ?: 'my default system property value'}") THIS IS HOW SPRING PROPOSE TO SET A DEFAULT VALUE
//#Value("#{systemProperties['some.key'] ?: #{configProperties['local.datasource.username']}}") THIS IS WHAT I WANT TO ACHIEVE, HOWEVER NOT COMPILING,
private String username;

What you are looking for are simple Property Palceholders.
While the Spring Expression Language supports the #{} syntax for rather complex expressions (ternary, calls, expressions, math), injecting property values and defaults is in most cases better done using a simple property placeholder ${value:defaultValue} approach:
#Property("${some.key:local.datasource.username}")
private String username;
Where some.key is being resolved (independent of its origin), and if that is null, Spring defaults to the value of local.datasource.username.
Please keep in mind, that even if some.key is present, Spring will throw an exception when it can't resolve your default property.
See also:
Spring Expression Language (SpEL) with #Value: dollar vs. hash ($ vs. #) and
A Quick Guide to Spring #Value

How to get the list of properties retrieved in #Value inside my Spring Application

In my application, I have dozen of properties mapped to my bean attributes like that:
#Value("#{props['prop1']}")
private String prop1;
#Value("#{props['prop2']}")
private String prop2;
#Value("#{props['prop3']}")
private String prop3;
In my property file, I always need to define the values:
prop1=value1
prop2=value2
prop3=specific-value
It's heavy because most of the properties are often the same. I would prefer to define default values in my code and then override what I need in my property file. The code can looks like this:
#Value("#{props['prop1'] ?: value1}")
private String prop1;
#Value("#{props['prop2'] ?: value2}")
private String prop2;
#Value("#{props['prop3'] ?: value3}")
private String prop3;
And in my property file, I just change prop3:
prop3=specific-value
This is handy because we have a smaller property file that is easier to maintain. Spring boot can also help more by picking the right property file according to the enabled profile.
Now I have one concern: as a developer, how I can know all the configuration points inside my application? If we are forced to add all the properties to the property file, then we can have the list of supported properties easily. However with an approach where only a few properties are defined, we lose this benefit and it becomes tricky to know what property can be changed.
I'm curious to know how this issue has been already addressed before and how. I would expect to have a utility tool like a maven plugin that can scan my beans, detects inside them the attributes annotated with #Value and prints a table of all properties and their default value defined in the application.

How to change any environment property value using Spring

Just as we can read the property using #Value annotation, is there any way to change the value of the property through the code?
#Value("#{envProps['" + Keys.Env.updatedDate + "']}")
private String date;
value in environment.properties
updatedDate =2013-10-01
I want to change the value to 2013-10-16. How to do it?
Thanks

Sorry you cant do that
#Annotations and its values in java are constants.
the #Annotations are stored in class file (at RuntimeVisibleAnnotations) while compiling.
the values of them can be String, Class, or other value and can't be change at Runtime.
see jvm class file spec
Alternative solution
use a place holder like
#Value("${updatedDate.envProps}")
private String date;
and write your custom PropertyPlaceholderConfigurer to parse updatedDate.envProps to the value you want