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Spring boot Load configuration per environment using PropertySource

I'm trying to add authorization to several microservices. Given all the services share similar authorization process, I want to extract the logic to a shared library.
I managed to create library, but I realise all the configurations need to be set in the application.yml file in the microservice which calls the library. I don't want to expose some of the configurations at service layer though.
After some searches, I found I could set #PropertySource("library.properties") in my library's configuration class to force reading properties from the specified .properties file within the library.
The problem now is I want to set different values for different environments, e.g. authorization URL for test and production would be different. How can I configure the file so that the configuration class would read same property value based on active profile (e.g. environment = test/staging/production)?

You can have multiple property files such as "application-environment.yml” in your resource folder. Spring framework picks the right one based on the active profile.
For example, if you define a “staging” environment and have a staging profile and then your property file should be named as application-staging.yml.

Set application specific properties within the application

Is there any way by which we can set application specific properties within the application? Like server port. Searching solution other than Spring boot and Server.xml. I am using Spring MVC in my web application. Any help is highly appreciated.

You need to use -D option to specify the explicit configuration parameters if you are executing from command line as follows.
java -Dyour_config_param="value" -jar your_app.jar

Have a look at Spring application properties: https://www.tutorialspoint.com/spring_boot/spring_boot_application_properties.htm.
You can also have multiple properties files for your different environments such as E1, E2 or E3 but appending your properties file name like application-e1.properties and then passing the --Dspring.profiles.active=e1 flag to your application.

You can declare property sources in your application.
Here you have a Spring (not Spring Boot) specific blog entry explaining all you need to know about property management:
https://spring.io/blog/2011/02/15/spring-3-1-m1-unified-property-management/
Also, you can check some alternative solutions in this related question:
How to read values from properties file?

How to secure liquibase properties (database password etc.) in springboot application?

While developing a springboot-liquibase application following this I need to specify the database username + password as liquibase.user and liquibase.password in the application.properties file. I am looking for a better secure way to use these parameter (dynamically fetched from some other place and use inside my java code)
Is there a way to achieve this?

There are couple of things you can do:
You can encrypt you properties file using jasypt-spring-boot. For more details have a look at demo app
If you are developing distributed system, then spring-cloud-config provides server and client-side support for externalized configuration in a distributed system. With the Config Server you have a central place to manage external properties for applications across all environments

Spring Cloud Config
This project allows you to use an external, centralized configuration repository for one or more applications. You don't need to rebuild your application if a property changes. You can simply change the property in your configuration repository and even push the changes to all of your applications.
See this Getting Started Guide.

How to config routes in Spring Cloud Zuul

In Spring Cloud Zuul server we can define all routes which need be redirected via "application.properties".
For example:
zuul.routes.resource.path=/api/**
zuul.routes.resource.url=http://api.com:9025
Once the fat jar is created the "application.properties" is encapsulated into jar, and it's not possible to edit and reload the rules inside the file.
Is there any to inform Zuul about the routes in an external file, and at the same time reload them without stopping the server?

You can use spring cloud config for that.
Spring Cloud Config provides server and client-side support for externalized configuration in a distributed system. With the Config Server you have a central place to manage external properties for applications across all environments
http://cloud.spring.io/spring-cloud-config/spring-cloud-config.html#_spring_cloud_config_client

...a bit late to the party, but...
You can do all that you want with the ConfigServer!
Create an application.yml for config that is common across ALL applications
Create profile specific application-mycommonprofile.yml. As 1. but for the 'mycommonprofile' profile.
Create an appX.yml for each application that is specific to that application.
Create profile specific appX-myprofile.yml. As 3. but for the 'myprofile' profile.
All of these files are optional and are not dependent on any others. You can have an application-mycommonprofile.yml without an application.yml for example.
Hope that helps!

Another late-to-the-party answer, but another way is to use a profile config file, which lives in the filesystem, outside the fatjar.
If the configuration name of your Zuul proxy is 'zuul' and your normal config file is 'zuul.properties' or 'zuul.yaml', then it looks for a profile-specific config file in 'zuul-.properties' or 'zuul-.yaml'.
If you do not specify a profile, then the profile named 'default' is active.
So you can load properties from an external file name 'zuul-default.properties' or 'zuul-default.yaml' (or 'zuul-default.yml', if you use a 3-letter filename extension).
This will then be loaded when no other profile is specified.

How do you modify values in java properties file

I have a config.properties file which contains configurable properties e.g. database connection details in a webapp deployed on tomcat. e.g.
local.driver: com.mysql.jdbc.Driver
local.username:myuser
local.password:mypass
dev.driver: com.mysql.jdbc.Driver
dev.username:mydevuser
dev.password:mydevpass
I can retrieve the values from config.properties using spring Environment object or #Value.
My question is how do you make Spring's environment object pick local properties when running on local and dev properties when running on dev? Also it doesn't sound right to save sensitive data e.g. production database connection
details in properties file which will float around in code base. So how do you add production detail when in production environment? Ideally I would want to change them as and when I like and not have to redeploy the app. Am I going the right direction?
Note - All the answers I have seen on SO discuss changing these properties within java code. I don't want to do that I want to be able to configure these values independent of the application code.
Thanks

You can have a look at spring profiles to load a specific file for a specific environment.
Alternatively, you can also parameterize the file from where the properties are loaded in the application context using a JNDI property or an environment property set in the container.
Example:
<context:property-placeholder ignore-unresolvable="true" location="${env.config.file:classpath*:META-INF/spring/profiles/dev/dev.properties}" />
The env.config.file can be set at the container level (say Tomcat) using -Denv.config.file= when starting it. By doing this, Spring automagically finds the property in the system props and replaces it. If you don't set it explicitly (for example, in dev where you might use some other container, such as jetty), it would use the given default value (in this example, dev.properties).
By putting the properties files outside the war / ear, they can be changed at will, and only the context needs to be restarted. Alternatively, you could look into re-loadable property placeholders. This also helps if you don't want passwords stored in the war in clear.
For encrypting information in the property files, if you're using Spring 3, you can also check: http://www.jasypt.org/spring3.html.

for picking env specific values you have couple of options
If you can create multiple properties file based on env then you can use Spring profile feature (i.e spring.profiles.active), this will allow to control properties file to loaded via JVM parameter.
If you still want to keep all the stuff in single fle then you can override PropertyPlaceholderConfigurer to take env details from JVM parameter or default to some value
On security question , one the approach is to store encrypted password in prop file.