Document all predefined variables from ApplicationContext available in SpEL expressions

[quaff]

Currently the document only mentioned systemProperties as predefined variable, actually all builtin beans registered by AbstractApplicationContext are predefined variables, such as environment and systemEnvironment and messageSource and applicationEventMulticaster and lifecycleProcessor. It's worthy to add a new section describe those predefined variables.

[quaff]

Same for javadoc of StandardBeanExpressionResolver

[quaff]

https://github.com/spring-projects/spring-framework/blob/master/src/docs/asciidoc/core/core-expressions.adoc#the-elvis-operator

ExpressionParser parser = new SpelExpressionParser();

String name = parser.parseExpression("name?:'Unknown'").getValue(String.class);
System.out.println(name);  // 'Unknown'

It throws exception instead of print Unknown because no variable name defined

Exception in thread "main" org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'name' cannot be found on null
	at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:213)
	at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:104)
	at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:91)
	at org.springframework.expression.spel.ast.Elvis.getValueInternal(Elvis.java:53)
	at org.springframework.expression.spel.ast.SpelNodeImpl.getTypedValue(SpelNodeImpl.java:117)
	at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:177)

[sbrannen]

It throws exception instead of print Unknown because no variable name defined

Can you please open a separate issue for that?

[quaff]

It throws exception instead of print Unknown because no variable name defined

Can you please open a separate issue for that?

@sbrannen I've created #25045

[jhoeller]

I've revised the entire section a bit, adding a note that generally all beans in the context are available as predefined variables in SpEL expressions, including standard context beans... singling out environment, systemProperties and systemEnvironment (including their exposed Java type). This should cover all common cases since SPI beans in the context are really not meant to be commonly accessed in SpEL expressions.