spring学习7-PropertySource Abstraction

PropertySource Abstraction

Spring’s Environment abstraction provides search operations over a configurable hierarchy of property sources. To explain fully, consider the following:

ApplicationContext ctx = new GenericApplicationContext();
Environment env = ctx.getEnvironment();
boolean containsFoo = env.containsProperty("foo");
System.out.println("Does my environment contain the 'foo' property? " + containsFoo);

In the snippet above, we see a high-level way of asking Spring whether the

foo

property
is defined for the current environment. To answer this question, the

Environment

object
performs a search over a set of

PropertySource

objects.
A

PropertySource

is
a simple abstraction over any source of key-value pairs, and Spring’s

StandardEnvironment

is
configured with two PropertySource objects — one representing the set of JVM system properties (a la

System.getProperties()

)
and one representing the set of system environment variables (a la

System.getenv()

).

These default property sources are present for StandardEnvironment , for use in standalone applications. StandardServletEnvironment is populated with additional default property sources including servlet config and servlet context parameters. StandardPortletEnvironment similarly has access to portlet config and portlet context parameters as property sources. Both can optionally enable a JndiPropertySource . See Javadoc for details.

Concretely, when using the

StandardEnvironment

,
the call to

env.containsProperty("foo")

will
return true if a

foo

system
property or

foo

environment
variable is present at runtime.

The search performed is hierarchical. By default, system properties have precedence over environment variables, so if the foo property happens to be set in both places during a call to env.getProperty("foo") , the system property value will win and be returned preferentially over the environment variable.

Most importantly, the entire mechanism is configurable. Perhaps you have a custom source of properties that you’d like to integrate into this search. No problem — simply implement and instantiate your own

PropertySource

and
add it to the set of

PropertySources

for
the current

Environment

:

ConfigurableApplicationContext ctx = new GenericApplicationContext();
MutablePropertySources sources = ctx.getEnvironment().getPropertySources();
sources.addFirst(new MyPropertySource());

In the code above,

MyPropertySource

has
been added with highest precedence in the search. If it contains a

foo

property,
it will be detected and returned ahead of any

foo

property
in any other

PropertySource

.
The

MutablePropertySources

API
exposes a number of methods that allow for precise manipulation of the set of property sources.

5.13.4 @PropertySource

The

@PropertySource

annotation
provides a convenient and declarative mechanism for adding a

PropertySource

to
Spring’s

Environment

.
Given a file "app.properties" containing the key/value pair

testbean.name=myTestBean

,
the following

@Configuration

class
uses

@PropertySource

in
such a way that a call to

testBean.getName()

will
return "myTestBean".

@Configuration
@PropertySource("classpath:/com/myco/app.properties")
public class AppConfig {
@Autowired
Environment env;

@Bean
public TestBean testBean() {
TestBean testBean = new TestBean();
testBean.setName(env.getProperty("testbean.name"));
return testBean;
}
}

Any

${...}

placeholders
present in a

@PropertySource

resource
location will be resolved against the set of property sources already registered against the environment. For example:

@Configuration
@PropertySource("classpath:/com/${my.placeholder:default/path}/app.properties")
public class AppConfig {
@Autowired
Environment env;

@Bean
public TestBean testBean() {
TestBean testBean = new TestBean();
testBean.setName(env.getProperty("testbean.name"));
return testBean;
}
}

Assuming that "my.placeholder" is present in one of the property sources already registered, e.g. system properties or environment variables, the placeholder will be resolved to the corresponding value. If not,
then "default/path" will be used as a default. If no default is specified and a property cannot be resolved, an

IllegalArgumentException

will
be thrown.