Spring Expression Language - Java 8 forEach或stream on list

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问题描述

SpEL中列表中的stream或forEach是否可能？
例如。

Is it possible for stream or forEach on list in SpEL?e.g.

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))

推荐答案

SpEL是不是Java，它是一种不同的语言;首字母缩略词代表Spring Expression 语言。

SpEL is not Java, it's a different language; the acronym stands for Spring Expression Language.

它不理解Java8 lambdas所以无法解析 x - > ... 。

It doesn't understand Java8 lambdas so can't parse x -> ....

此外，使用T运算符调用静态方法。

Also, static methods are invoked with the T operator.

所以，这有效......

So, this works...

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）。

您可以使用流，但只能使用不使用lambdas的简单方法...

You can use streams, but only with simple methods that don't take lambdas...

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

或

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

编辑

但是，你可以将lambdas注册为SpEL #functions ，所以这样可以正常工作......

You can, however, register lambdas as SpEL #functions, so this works fine...

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，更一般地说......

Or, more generally...

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;
    }

}

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