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Revise SpEL Evaluation documentation

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Sam Brannen 2024-01-09 12:03:28 +01:00
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framework-docs/modules/ROOT/pages/core/expressions/evaluation.adoc
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@ -1,12 +1,12 @@
[[expressions-evaluation]]
= Evaluation
This section introduces the simple use of SpEL interfaces and its expression language.
The complete language reference can be found in
This section introduces programmatic use of SpEL's interfaces and its expression language.
The complete language reference can be found in the
xref:core/expressions/language-ref.adoc[Language Reference].
The following code introduces the SpEL API to evaluate the literal string expression,
`Hello World`.
The following code demonstrates how to use the SpEL API to evaluate the literal string
expression, `Hello World`.
[tabs]
======
@ -31,21 +31,21 @@ Kotlin::
<1> The value of the message variable is `'Hello World'`.
======
The SpEL classes and interfaces you are most likely to use are located in the
`org.springframework.expression` package and its sub-packages, such as `spel.support`.
The `ExpressionParser` interface is responsible for parsing an expression string. In
the preceding example, the expression string is a string literal denoted by the surrounding single
quotation marks. The `Expression` interface is responsible for evaluating the previously defined
expression string. Two exceptions that can be thrown, `ParseException` and
`EvaluationException`, when calling `parser.parseExpression` and `exp.getValue`,
respectively.
The `ExpressionParser` interface is responsible for parsing an expression string. In the
preceding example, the expression string is a string literal denoted by the surrounding
single quotation marks. The `Expression` interface is responsible for evaluating the
defined expression string. The two types of exceptions that can be thrown when calling
`parser.parseExpression(...)` and `exp.getValue(...)` are `ParseException` and
`EvaluationException`, respectively.
SpEL supports a wide range of features, such as calling methods, accessing properties,
SpEL supports a wide range of features such as calling methods, accessing properties,
and calling constructors.
In the following example of method invocation, we call the `concat` method on the string literal:
In the following method invocation example, we call the `concat` method on the string
literal, `Hello World`.
[tabs]
======
@ -70,7 +70,8 @@ Kotlin::
<1> The value of `message` is now 'Hello World!'.
======
The following example of calling a JavaBean property calls the `String` property `Bytes`:
The following example demonstrates how to access the `Bytes` JavaBean property of the
string literal, `Hello World`.
[tabs]
======
@ -100,10 +101,10 @@ Kotlin::
======
SpEL also supports nested properties by using the standard dot notation (such as
`prop1.prop2.prop3`) and also the corresponding setting of property values.
`prop1.prop2.prop3`) as well as the corresponding setting of property values.
Public fields may also be accessed.
The following example shows how to use dot notation to get the length of a literal:
The following example shows how to use dot notation to get the length of a string literal.
[tabs]
======
@ -133,7 +134,7 @@ Kotlin::
======
The String's constructor can be called instead of using a string literal, as the following
example shows:
example shows.
[tabs]
======
@ -145,7 +146,7 @@ Java::
Expression exp = parser.parseExpression("new String('hello world').toUpperCase()"); // <1>
String message = exp.getValue(String.class);
----
<1> Construct a new `String` from the literal and make it be upper case.
<1> Construct a new `String` from the literal and convert it to upper case.
Kotlin::
+
@ -155,10 +156,9 @@ Kotlin::
val exp = parser.parseExpression("new String('hello world').toUpperCase()") // <1>
val message = exp.getValue(String::class.java)
----
<1> Construct a new `String` from the literal and make it be upper case.
<1> Construct a new `String` from the literal and convert it to upper case.
======
Note the use of the generic method: `public <T> T getValue(Class<T> desiredResultType)`.
Using this method removes the need to cast the value of the expression to the desired
result type. An `EvaluationException` is thrown if the value cannot be cast to the
@ -166,8 +166,8 @@ type `T` or converted by using the registered type converter.
The more common usage of SpEL is to provide an expression string that is evaluated
against a specific object instance (called the root object). The following example shows
how to retrieve the `name` property from an instance of the `Inventor` class or
create a boolean condition:
how to retrieve the `name` property from an instance of the `Inventor` class and how to
reference the `name` property in a boolean expression.
[tabs]
======
@ -240,7 +240,7 @@ It excludes Java type references, constructors, and bean references. It also req
you to explicitly choose the level of support for properties and methods in expressions.
By default, the `create()` static factory method enables only read access to properties.
You can also obtain a builder to configure the exact level of support needed, targeting
one or some combination of the following:
one or some combination of the following.
* Custom `PropertyAccessor` only (no reflection)
* Data binding properties for read-only access
@ -252,16 +252,15 @@ one or some combination of the following:
By default, SpEL uses the conversion service available in Spring core
(`org.springframework.core.convert.ConversionService`). This conversion service comes
with many built-in converters for common conversions but is also fully extensible so that
you can add custom conversions between types. Additionally, it is
generics-aware. This means that, when you work with generic types in
expressions, SpEL attempts conversions to maintain type correctness for any objects
it encounters.
with many built-in converters for common conversions, but is also fully extensible so
that you can add custom conversions between types. Additionally, it is generics-aware.
This means that, when you work with generic types in expressions, SpEL attempts
conversions to maintain type correctness for any objects it encounters.
What does this mean in practice? Suppose assignment, using `setValue()`, is being used
to set a `List` property. The type of the property is actually `List<Boolean>`. SpEL
recognizes that the elements of the list need to be converted to `Boolean` before
being placed in it. The following example shows how to do so:
being placed in it. The following example shows how to do so.
[tabs]
======
@ -325,7 +324,7 @@ constructor before setting the specified value. If the element type does not hav
default constructor, `null` will be added to the array or list. If there is no built-in
or custom converter that knows how to set the value, `null` will remain in the array or
list at the specified index. The following example demonstrates how to automatically grow
the list:
the list.
[tabs]
======
@ -394,11 +393,11 @@ xref:appendix.adoc#appendix-spring-properties[Supported Spring Properties]).
[[expressions-spel-compilation]]
== SpEL Compilation
Spring Framework 4.1 includes a basic expression compiler. Expressions are usually
interpreted, which provides a lot of dynamic flexibility during evaluation but
does not provide optimum performance. For occasional expression usage,
this is fine, but, when used by other components such as Spring Integration,
performance can be very important, and there is no real need for the dynamism.
Spring provides a basic compiler for SpEL expressions. Expressions are usually
interpreted, which provides a lot of dynamic flexibility during evaluation but does not
provide optimum performance. For occasional expression usage, this is fine, but, when
used by other components such as Spring Integration, performance can be very important,
and there is no real need for the dynamism.
The SpEL compiler is intended to address this need. During evaluation, the compiler
generates a Java class that embodies the expression behavior at runtime and uses that
@ -411,16 +410,17 @@ information can cause trouble later if the types of the various expression eleme
change over time. For this reason, compilation is best suited to expressions whose
type information is not going to change on repeated evaluations.
Consider the following basic expression:
Consider the following basic expression.
[source,java,indent=0,subs="verbatim,quotes"]
----
someArray[0].someProperty.someOtherProperty < 0.1
someArray[0].someProperty.someOtherProperty < 0.1
----
Because the preceding expression involves array access, some property de-referencing,
and numeric operations, the performance gain can be very noticeable. In an example
micro benchmark run of 50000 iterations, it took 75ms to evaluate by using the
interpreter and only 3ms using the compiled version of the expression.
Because the preceding expression involves array access, some property de-referencing, and
numeric operations, the performance gain can be very noticeable. In an example micro
benchmark run of 50,000 iterations, it took 75ms to evaluate by using the interpreter and
only 3ms using the compiled version of the expression.
[[expressions-compiler-configuration]]
@ -428,33 +428,34 @@ interpreter and only 3ms using the compiled version of the expression.
The compiler is not turned on by default, but you can turn it on in either of two
different ways. You can turn it on by using the parser configuration process
(xref:core/expressions/evaluation.adoc#expressions-parser-configuration[discussed earlier]) or by using a Spring property
when SpEL usage is embedded inside another component. This section discusses both of
these options.
(xref:core/expressions/evaluation.adoc#expressions-parser-configuration[discussed
earlier]) or by using a Spring property when SpEL usage is embedded inside another
component. This section discusses both of these options.
The compiler can operate in one of three modes, which are captured in the
`org.springframework.expression.spel.SpelCompilerMode` enum. The modes are as follows:
`org.springframework.expression.spel.SpelCompilerMode` enum. The modes are as follows.
* `OFF` (default): The compiler is switched off.
* `IMMEDIATE`: In immediate mode, the expressions are compiled as soon as possible. This
is typically after the first interpreted evaluation. If the compiled expression fails
(typically due to a type changing, as described earlier), the caller of the expression
evaluation receives an exception.
* `MIXED`: In mixed mode, the expressions silently switch between interpreted and compiled
mode over time. After some number of interpreted runs, they switch to compiled
form and, if something goes wrong with the compiled form (such as a type changing, as
described earlier), the expression automatically switches back to interpreted form
again. Sometime later, it may generate another compiled form and switch to it. Basically,
the exception that the user gets in `IMMEDIATE` mode is instead handled internally.
is typically after the first interpreted evaluation. If the compiled expression fails
(typically due to a type changing, as described earlier), the caller of the expression
evaluation receives an exception.
* `MIXED`: In mixed mode, the expressions silently switch between interpreted and
compiled mode over time. After some number of interpreted runs, they switch to compiled
form and, if something goes wrong with the compiled form (such as a type changing, as
described earlier), the expression automatically switches back to interpreted form
again. Sometime later, it may generate another compiled form and switch to it.
Basically, the exception that the user gets in `IMMEDIATE` mode is instead handled
internally.
`IMMEDIATE` mode exists because `MIXED` mode could cause issues for expressions that
have side effects. If a compiled expression blows up after partially succeeding, it
may have already done something that has affected the state of the system. If this
has happened, the caller may not want it to silently re-run in interpreted mode,
since part of the expression may be running twice.
since part of the expression may be run twice.
After selecting a mode, use the `SpelParserConfiguration` to configure the parser. The
following example shows how to do so:
following example shows how to do so.
[tabs]
======
@ -491,15 +492,16 @@ Kotlin::
----
======
When you specify the compiler mode, you can also specify a classloader (passing null is allowed).
Compiled expressions are defined in a child classloader created under any that is supplied.
It is important to ensure that, if a classloader is specified, it can see all the types involved in
the expression evaluation process. If you do not specify a classloader, a default classloader is used
(typically the context classloader for the thread that is running during expression evaluation).
When you specify the compiler mode, you can also specify a `ClassLoader` (passing `null`
is allowed). Compiled expressions are defined in a child `ClassLoader` created under any
that is supplied. It is important to ensure that, if a `ClassLoader` is specified, it can
see all the types involved in the expression evaluation process. If you do not specify a
`ClassLoader`, a default `ClassLoader` is used (typically the context `ClassLoader` for
the thread that is running during expression evaluation).
The second way to configure the compiler is for use when SpEL is embedded inside some
other component and it may not be possible to configure it through a configuration
object. In these cases, it is possible to set the `spring.expression.compiler.mode`
object. In such cases, it is possible to set the `spring.expression.compiler.mode`
property via a JVM system property (or via the
xref:appendix.adoc#appendix-spring-properties[`SpringProperties`] mechanism) to one of the
`SpelCompilerMode` enum values (`off`, `immediate`, or `mixed`).
@ -508,18 +510,14 @@ xref:appendix.adoc#appendix-spring-properties[`SpringProperties`] mechanism) to
[[expressions-compiler-limitations]]
=== Compiler Limitations
Since Spring Framework 4.1, the basic compilation framework is in place. However, the framework
does not yet support compiling every kind of expression. The initial focus has been on the
common expressions that are likely to be used in performance-critical contexts. The following
kinds of expression cannot be compiled at the moment:
Spring does not support compiling every kind of expression. The primary focus is on
common expressions that are likely to be used in performance-critical contexts. The
following kinds of expressions cannot be compiled.
* Expressions involving assignment
* Expressions relying on the conversion service
* Expressions using custom resolvers or accessors
* Expressions using selection or projection
More types of expressions will be compilable in the future.
Compilation of additional kinds of expressions may be supported in the future.