现在回答已经太晚了，但我也遇到过同样的情况，在java中计算表达式，这可能会帮助到一些人

MVEL对表达式进行运行时求值，我们可以在String中编写java代码来得到它的值。

    String expressionStr = "x+y";
    Map<String, Object> vars = new HashMap<String, Object>();
    vars.put("x", 10);
    vars.put("y", 20);
    ExecutableStatement statement = (ExecutableStatement) MVEL.compileExpression(expressionStr);
    Object result = MVEL.executeExpression(statement, vars);

现在回答已经太晚了，但我也遇到过同样的情况，在java中计算表达式，这可能会帮助到一些人

MVEL对表达式进行运行时求值，我们可以在String中编写java代码来得到它的值。

    String expressionStr = "x+y";
    Map<String, Object> vars = new HashMap<String, Object>();
    vars.put("x", 10);
    vars.put("y", 20);
    ExecutableStatement statement = (ExecutableStatement) MVEL.compileExpression(expressionStr);
    Object result = MVEL.executeExpression(statement, vars);

这实际上是对@Boann给出的答案的补充。它有一个轻微的错误，导致“-2 ^ 2”给出一个错误的结果-4.0。这里的问题是求幂的点。只需将取幂运算移到parseTerm()的块中，就可以了。看看下面，这是@Boann的回答略有修改。修改意见见评论。

public static double eval(final String str) {
    return new Object() {
        int pos = -1, ch;

        void nextChar() {
            ch = (++pos < str.length()) ? str.charAt(pos) : -1;
        }

        boolean eat(int charToEat) {
            while (ch == ' ') nextChar();
            if (ch == charToEat) {
                nextChar();
                return true;
            }
            return false;
        }

        double parse() {
            nextChar();
            double x = parseExpression();
            if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
            return x;
        }

        // Grammar:
        // expression = term | expression `+` term | expression `-` term
        // term = factor | term `*` factor | term `/` factor
        // factor = `+` factor | `-` factor | `(` expression `)`
        //        | number | functionName factor | factor `^` factor

        double parseExpression() {
            double x = parseTerm();
            for (;;) {
                if      (eat('+')) x += parseTerm(); // addition
                else if (eat('-')) x -= parseTerm(); // subtraction
                else return x;
            }
        }

        double parseTerm() {
            double x = parseFactor();
            for (;;) {
                if      (eat('*')) x *= parseFactor(); // multiplication
                else if (eat('/')) x /= parseFactor(); // division
                else if (eat('^')) x = Math.pow(x, parseFactor()); //exponentiation -> Moved in to here. So the problem is fixed
                else return x;
            }
        }

        double parseFactor() {
            if (eat('+')) return parseFactor(); // unary plus
            if (eat('-')) return -parseFactor(); // unary minus

            double x;
            int startPos = this.pos;
            if (eat('(')) { // parentheses
                x = parseExpression();
                eat(')');
            } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
                while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
                x = Double.parseDouble(str.substring(startPos, this.pos));
            } else if (ch >= 'a' && ch <= 'z') { // functions
                while (ch >= 'a' && ch <= 'z') nextChar();
                String func = str.substring(startPos, this.pos);
                x = parseFactor();
                if (func.equals("sqrt")) x = Math.sqrt(x);
                else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
                else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
                else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
                else throw new RuntimeException("Unknown function: " + func);
            } else {
                throw new RuntimeException("Unexpected: " + (char)ch);
            }

            //if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation -> This is causing a bit of problem

            return x;
        }
    }.parse();
}

对于JDK1.6，您可以使用内置的Javascript引擎。

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;

public class Test {
  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");
    String foo = "40+2";
    System.out.println(engine.eval(foo));
    } 
}

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class test2 {
    public static void main(String[] args) throws ScriptException {
        String s = "10+2";
        ScriptEngineManager mn = new ScriptEngineManager();
        ScriptEngine en = mn.getEngineByName("js");
        Object result = en.eval(s);
        System.out.println(result);
    }
}

另一种方法是使用Spring表达式语言或SpEL，它在计算数学表达式时做了更多的工作，因此可能会有点过度。您不必使用Spring框架来使用这个表达式库，因为它是独立的。从SpEL文档中复制示例:

ExpressionParser parser = new SpelExpressionParser();
int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2 
double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); //24.0