本文讨论了各种方法。以下是文中提到的两种关键方法:

Apache的JEXL

允许脚本包含对java对象的引用。

// Create or retrieve a JexlEngine
JexlEngine jexl = new JexlEngine();
// Create an expression object
String jexlExp = "foo.innerFoo.bar()";
Expression e = jexl.createExpression( jexlExp );
 
// Create a context and add data
JexlContext jctx = new MapContext();
jctx.set("foo", new Foo() );
 
// Now evaluate the expression, getting the result
Object o = e.evaluate(jctx);

使用JDK中嵌入的javascript引擎:

private static void jsEvalWithVariable()
{
    List<String> namesList = new ArrayList<String>();
    namesList.add("Jill");
    namesList.add("Bob");
    namesList.add("Laureen");
    namesList.add("Ed");
 
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine jsEngine = mgr.getEngineByName("JavaScript");
 
    jsEngine.put("namesListKey", namesList);
    System.out.println("Executing in script environment...");
    try
    {
      jsEngine.eval("var x;" +
                    "var names = namesListKey.toArray();" +
                    "for(x in names) {" +
                    "  println(names[x]);" +
                    "}" +
                    "namesListKey.add(\"Dana\");");
    }
    catch (ScriptException ex)
    {
        ex.printStackTrace();
    }
}

这样怎么样:

String st = "10+3";
int result;
for(int i=0;i<st.length();i++)
{
  if(st.charAt(i)=='+')
  {
    result=Integer.parseInt(st.substring(0, i))+Integer.parseInt(st.substring(i+1, st.length()));
    System.out.print(result);
  }         
}

并相应地对其他数学运算符做类似的事情。

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);
    }
}

使用带有代码注入处理的JDK1.6 Javascript引擎尝试下面的示例代码。

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

public class EvalUtil {
private static ScriptEngine engine = new ScriptEngineManager().getEngineByName("JavaScript");
public static void main(String[] args) {
    try {
        System.out.println((new EvalUtil()).eval("(((5+5)/2) > 5) || 5 >3 "));
        System.out.println((new EvalUtil()).eval("(((5+5)/2) > 5) || true"));
    } catch (Exception e) {
        e.printStackTrace();
    }
}
public Object eval(String input) throws Exception{
    try {
        if(input.matches(".*[a-zA-Z;~`#$_{}\\[\\]:\\\\;\"',\\.\\?]+.*")) {
            throw new Exception("Invalid expression : " + input );
        }
        return engine.eval(input);
    } catch (Exception e) {
        e.printStackTrace();
        throw e;
    }
 }
}

你可以看看Symja框架:

ExprEvaluator util = new ExprEvaluator(); 
IExpr result = util.evaluate("10-40");
System.out.println(result.toString()); // -> "-30" 

请注意，可以计算更复杂的表达式:

// D(...) gives the derivative of the function Sin(x)*Cos(x)
IAST function = D(Times(Sin(x), Cos(x)), x);
IExpr result = util.evaluate(function);
// print: Cos(x)^2-Sin(x)^2