Spring Expression Language - Java 8 forEach或stream on list

Question

是否可以在SpEL中列出stream或forEach？例如

List<String> x = new LinkedList<>(Arrays.asList("A","AAB"));
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext(x);
parser.parseExpression("x.stream().map(x -> x.replaceAll(\"A\", \"B\")).collect(Collectors.toList())").getValue(context))

Answer 1

SpEL不是Java,它是一种不同的语言; 首字母缩写词代表Spring Expression Language.

它不懂Java8 lambdas所以无法解析x -> ....

此外,使用T运算符调用静态方法.

所以,这有效......

List<String> x = new LinkedList<>(Arrays.asList("A","AAB"));
ExpressionParser parser = new SpelExpressionParser();
Expression expression = parser.parseExpression("stream().collect(T(java.util.stream.Collectors).toList())");
System.out.println(expression.getValue(x));

(但它不是很有用).

你可以使用流,但只能使用不带lambdas的简单方法...

Expression expression = parser.parseExpression("stream().findFirst().get()");
Expression expression = parser.parseExpression("stream().count()");

要么

List<String> x = new LinkedList<>(Arrays.asList("A","AAB", "A"));
ExpressionParser parser = new SpelExpressionParser();
Expression expression = parser.parseExpression("stream().distinct().collect(T(java.util.stream.Collectors).toList())");
System.out.println(expression.getValue(x));

等等

编辑

但是,你可以将lambda注册为SpEL #functions,所以这很好......

public class So48840190Application {

    public static void main(String[] args) throws Exception {
        List<String> x = new LinkedList<>(Arrays.asList("A","AAB", "A"));
        ExpressionParser parser = new SpelExpressionParser();
        StandardEvaluationContext ec = new StandardEvaluationContext();
        ec.registerFunction("aToB", So48840190Application.class.getMethod("aToB"));
        Expression expression = parser.parseExpression(
                "stream().map(#aToB()).collect(T(java.util.stream.Collectors).toList())");
        System.out.println(expression.getValue(ec, x));
    }

    public static Function<String, String> aToB() {
        return s -> s.replaceAll("A", "B");
    }

}

和

[B, BBB, B]

EDIT2

或者,更一般地......

public class So48840190Application {

    public static void main(String[] args) throws Exception {
        List<String> x = new LinkedList<>(Arrays.asList("A","AAB", "A"));
        ExpressionParser parser = new SpelExpressionParser();
        StandardEvaluationContext ec = new StandardEvaluationContext();
        ec.registerFunction("replaceAll",
                So48840190Application.class.getMethod("replaceAll", String.class, String.class));
        ec.registerFunction("toLowerCase",
                So48840190Application.class.getMethod("toLowerCase"));
        Expression expression = parser.parseExpression(
                "stream().map(#replaceAll('A', 'B')).map(#toLowerCase()).collect(T(java.util.stream.Collectors).toList())");
        System.out.println(expression.getValue(ec, x));
    }

    public static Function<String, String> replaceAll(String from, String to) {
        return s -> s.replaceAll(from, to);
    }

    public static Function<String, String> toLowerCase() {
        return String::toLowerCase;
    }

}