Relaxed Binding 2.0

Madhura Bhave edited this page Jan 31, 2018 · 4 revisions
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Spring Boot Relaxed Binding 2.0

The purpose of this page is to describe in detail the relaxed rules for binding properties to the environment as well as reading them from the environment.

Setting properties in the environment

Spring Boot allows you to configure properties for your application using properties files, YAML files, environment variables and command-line arguments.

Input Sources

Properties Files

Simple Types

Simple properties are bound by removing any special characters and converting to lowercase. For example, the following properties all result in the mapping spring.jpa.databaseplatform=mysql:

spring.jpa.database-platform=mysql
spring.jpa.databasePlatform=mysql
spring.JPA.database_platform=mysql
Note
We recommend that properties are stored in lowercase kabab format. i.e. my.property-name=foo.

List Types

List types in properties files should be referenced using [ ] notation:

spring.my-example.url[0]=http://example.com
spring.my-example.url[1]=http://spring.io

Comma-separated values are also supported.

spring.my-example.urls=http://example.com,http://spring.io

Both of the mappings above result in the following properties:

spring.myexample.url[0]=http://example.com
spring.myexample.url[1]=http://spring.io

YAML Files

Simple Types

Simple YAML properties are bound by removing any special characters and converting to lowercase. For example, the following YAML properties all result in the mapping spring.jpa.databaseplatform=mysql:

spring:
  jpa:
    database-platform: mysql
    databasePlatform: mysql
    database_platform: mysql
Note
We recommend that properties are stored in yaml in lowercase kabab format. i.e. my.property-name=foo.

Lists

YAML list type may be specified in the standard or abbreviated form:

spring:
  my-example:
    url:
      - http://example.com
      - http://spring.io
spring:
  my-example:
    url: http://example.com, http://spring.io

Both are mapped as follows:

spring.myexample.url[0]=http://example.com
spring.myexample.url[1]=http://spring.io

Maps

Maps in yaml files may be specified in the standard form:

spring:
  my-example:
    foo: bar
    hello: world

This get mapped as:

spring.myexample.foo=bar
spring.myexample.hello=world

For map keys with non-alphanumeric characters (other than -) in them, surround the key name with []. For example:

spring:
  my-example:
    '[foo.baz]': bar

Environment Variables

Simple Types

Environment variables are bound by lowercasing and replacing _ with ..

For example: SPRING_JPA_DATABASEPLATFORM=mysql results in the property spring.jpa.databaseplatform=mysql.

Note
The _ delimiter must not be used within a property name. i.e. database-platform must be written as DATABASEPLATFORM and not DATABASE_PLATFORM.

Lists

The [ and ] characters cannot be used in environment variable names so instead a special form of _ is used. Any numeric value surrounded by underscores is converted to the [,] form. For example:

  • MY_FOO_1_ = my.foo[1]

  • MY_FOO_1_BAR = my.foo[1].bar

  • MY_FOO_1_2_ = my.foo[1][2]`

In addition, if an environment variable ends in a number the trailing _ may be omitted:

  • MY_FOO_1 = my.foo[1]

  • MY_FOO_1_2 = my.foo[1][2]`

System properties

Simple Types

System properties are bound by lowercasing and removing any special characters. For example, the following command line parameters will all result in spring.jpa.databaseplatform=mysql :

-Dspring.jpa.database-platform=mysql
-Dspring.jpa.databasePlatform=mysql
-Dspring.JPA.database_platform=mysql

List Types

List types in system properties should be referenced using [ ] notation:

-D"spring.my-example.url[0]=http://example.com"
-D"spring.my-example.url[1]=http://spring.io"

Comma-separated values are also supported:

-Dspring.my-example.url=http://example.com,http://spring.io

Both of the mappings above result in the following properties:

spring.myexample.url[0]=http://example.com
spring.myexample.url[1]=http://spring.io

Reading properties from the environment

If you read properties from the environment in your application, you will now need to use the uniform name for the property.

A uniform name
  • Is composed of elements separated in dots.

  • The last dot separates the prefix from the property name.

  • Must be alpha-numeric (a-z 0-9)

  • Must be lowercase

  • Hyphen to can be used to separate words.

  • The only other characters permitted are [ and ] which are used to indicate indexes.

  • Cannot start with a number.

For example a property as can be read from the environment as,

this.environment.containsProperty("spring.jpa.database-platform")
Note
Using the @Value annotation also requires specifying the properties in the uniform format.

Migrating from 1.x to 2.0

The new Binding API has replaced a lot of the old classes used for relaxed binding and relaxed property resolution.

  • The RelaxedDataBinder has been replaced by the Binder class. For example, the following POJO,

class FooProperties {

	private String bar;
	public String getBar() { ... }
    void setBar(String bar) { ... }
}
can be bound as follows:
Binder binder = Binder.get(environment);
FooProperties foo = binder.bind("foo", Bindable.of(FooProperties.class)).get();
Details about the bind method can be found in the javadoc.
  • The RelaxedPropertyResolver which was used to resolve properties in a relaxed way has also been removed. Instead, properties should be read directly from the environment using the uniform format:

this.environment.containsProperty("spring.jpa.database-platform")

Constraints

  • Collections must always be specified as a whole. Omitting indices will lead to an UnboundConfigurationPropertiesException For example, the following is not allowed,

foo[0] = a
foo[2] = b
  • Properties from non-enumerable property sources cannot be bound in a relaxed manner.

  • If a class annotated with @ConfigurationProperties needs to be validated, it must be annotated with @Validated.