Property Files in Mule 4

Main Points:

• Property files allow the centralization of properties.
• Property placeholders used instead of hard coding.
• Property file location must be configured in global elements.
• Property files can be constructed in Java or YAML format.

Centralization of Application Properties

It is best practice to centralize the management of properties rather than hard code values at the location of use. This allows for the reuse of values and better maintenance. Instead of changing the property in multiple places (if the value has been hardcoded), you change the value once in the property file and every location where that property placeholder is used gets to use the new value at runtime.

Setting up A Property File

There are three parts to set up when you want to use property files:

1. The property file in which the properties are defined.
2. The Configuration Property in the application's global elements.
3. The use of the property placeholders in the application configuration.

The Property File

The property file is a text file that contains the key/value pair representing the property value and its property placeholder name. The file can be either a Java property format file or a YAML file format.

Figure 1: Java-style property file format.

db.host=db.examples.com 
db.port=3306 
db.user=admin 
db.password=Password123 
db.database=products

Figure 2: YAML-style property fie format.

db:
  host: db.examples.com
  port: "3306"
  user: admin
  password: Password123
  database: products

Java-style property files have the .properties extend while the YAML-style property files have the .yaml file extension. The two formats in figures 1 and 2 defined the same properties. 

The Property File Location

The property files should be located on the classpath. Typically application property files are located in the src/main/resources directory. 

Provide a Configuration Property Setting

The Mule application must be configured with the location of the property file. This is done by providing its location in the Configuration Property configuration element in the global elements of the application. If the property file is call properties.yaml and is located on the classpath in the src/main/resources directory the configuration of the Configuration Property element looks like figure 3.

The Use of The Property Placeholders

Properties are used by referencing the name of the property enclosed in ${}. For example, the use of the properties set in figures 1 & 2 would look like this ${db.host}, ${db.port}, ${db.user}, ${db.password}, ${db.database} and be used as shown in figure 4.

It is also possible to reference Mule application properties using the Data Weave function Mule::p. 

Secure and Encrypted Properties

Typically, Mule applications require the use of secure properties such as passwords. These can be encrypted with an encryption key and used in the same way as insecure properties, they can also be hidden from view in CloudHub.