Variables

通过使用 #variableName 语法可以在表达式中引用变量。变量通过在 EvaluationContext 实现中使用 setVariable() 方法进行设置。

变量名称必须以字母（如下所定义）、下划线或美元符号开头。变量名称必须由以下受支持的字符类型中的一个或多个组成。

字母：返回 `true`的任何字符
- 这包括 A`到 `Z、a`到 `z、ü、`ñ`和 `é`等字母以及来自中文、日语、西里尔字母等其他字符集的字母。
digit: 0 to 9
underscore: _
dollar sign: $

在 EvaluationContext 中设置变量或根上下文对象时，建议变量或根上下文对象的类型为 public。否则，涉及非公共类型变量或根上下文对象的某些类型的 SpEL 表达式可能会无法评估或编译。

由于变量与评估上下文中的 functions 共享一个公共命名空间，因此必须小心确保变量名和函数名不会重叠。

以下示例展示了如何使用变量。

Java
Kotlin

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");

EvaluationContext context = SimpleEvaluationContext.forReadWriteDataBinding().build();
context.setVariable("newName", "Mike Tesla");

parser.parseExpression("name = #newName").getValue(context, tesla);
System.out.println(tesla.getName());  // "Mike Tesla"

val tesla = Inventor("Nikola Tesla", "Serbian")

val context = SimpleEvaluationContext.forReadWriteDataBinding().build()
context.setVariable("newName", "Mike Tesla")

parser.parseExpression("name = #newName").getValue(context, tesla)
println(tesla.name)  // "Mike Tesla"

The `#this` and `#root` Variables

#this 变量始终定义，并且引用当前评估对象（根据该对象解析不合格引用）。#root 变量始终定义，并且引用根上下文对象。虽然在评估表达式的组件时 #this 可能变化，但 #root 始终引用根。

以下示例显示如何将 #this 变量与 collection selection 结合使用。

Java
Kotlin

// Create a list of prime integers.
List<Integer> primes = List.of(2, 3, 5, 7, 11, 13, 17);

// Create parser and set variable 'primes' as the list of integers.
ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadWriteDataBinding().build();
context.setVariable("primes", primes);

// Select all prime numbers > 10 from the list (using selection ?{...}).
String expression = "#primes.?[#this > 10]";

// Evaluates to a list containing [11, 13, 17].
List<Integer> primesGreaterThanTen =
		parser.parseExpression(expression).getValue(context, List.class);

// Create a list of prime integers.
val primes = listOf(2, 3, 5, 7, 11, 13, 17)

// Create parser and set variable 'primes' as the list of integers.
val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadWriteDataBinding().build()
context.setVariable("primes", primes)

// Select all prime numbers > 10 from the list (using selection ?{...}).
val expression = "#primes.?[#this > 10]"

// Evaluates to a list containing [11, 13, 17].
val primesGreaterThanTen = parser.parseExpression(expression)
		.getValue(context) as List<Int>

以下示例显示如何将 #this 和 #root 变量与 collection projection 结合使用。

Java
Kotlin

// Create parser and evaluation context.
ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadWriteDataBinding().build();

// Create an inventor to use as the root context object.
Inventor tesla = new Inventor("Nikola Tesla");
tesla.setInventions("Telephone repeater", "Tesla coil transformer");

// Iterate over all inventions of the Inventor referenced as the #root
// object, and generate a list of strings whose contents take the form
// "<inventor's name> invented the <invention>." (using projection !{...}).
String expression = "#root.inventions.![#root.name + ' invented the ' + #this + '.']";

// Evaluates to a list containing:
// "Nikola Tesla invented the Telephone repeater."
// "Nikola Tesla invented the Tesla coil transformer."
List<String> results = parser.parseExpression(expression)
		.getValue(context, tesla, List.class);

// Create parser and evaluation context.
val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadWriteDataBinding().build()

// Create an inventor to use as the root context object.
val tesla = Inventor("Nikola Tesla")
tesla.setInventions("Telephone repeater", "Tesla coil transformer")

// Iterate over all inventions of the Inventor referenced as the #root
// object, and generate a list of strings whose contents take the form
// "<inventor's name> invented the <invention>." (using projection !{...}).
val expression = "#root.inventions.![#root.name + ' invented the ' + #this + '.']"

// Evaluates to a list containing:
// "Nikola Tesla invented the Telephone repeater."
// "Nikola Tesla invented the Tesla coil transformer."
val results = parser.parseExpression(expression)
		.getValue(context, tesla, List::class.java)

Variables

The #this and #root Variables

The `#this` and `#root` Variables