java字符串表达式解析器

Question

I was asked to include math expression inside a string say: "price: ${price}, tax: ${price}*${tax)"

the string is given at run-time and a Map values too

I used Velocity for this:

maven:

    <properties>
        <velocity.version>1.6.2</velocity.version>
        <velocity.tools.version>2.0</velocity.tools.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.apache.velocity</groupId>
            <artifactId>velocity</artifactId>
            <version>${velocity.version}</version>
        </dependency>
        <dependency>
            <groupId>org.apache.velocity</groupId>
            <artifactId>velocity-tools</artifactId>
            <version>${velocity.tools.version}</version>
        </dependency>
    </dependencies>

java:

public class VelocityUtils {
    public static String mergeTemplateIntoString(String template, Map<String,String> model)
    {
        try
        {
            final VelocityEngine ve = new VelocityEngine();
            ve.init();
            final VelocityContext context = new VelocityContext();
            context.put("math", new MathTool());
            context.put("number", new NumberTool());

            for (final Map.Entry<String, String> entry : model.entrySet())
            {
                final String macroName = entry.getKey();
               context.put(macroName, entry.getValue());
            }
            final StringWriter wr = new StringWriter();

            final String logStr = "";
            ve.evaluate(context, wr, logStr,template);

            return wr.toString();
        } catch(Exception e)
        {
            return "";
        }

    }
}

test class:

public class VelocityUtilsTest
{
    @Test
    public void testMergeTemplateIntoString() throws Exception
    {
        Map<String,String> model = new HashMap<>();
        model.put("price","100");
        model.put("tax","22");
        String parsedString = VelocityUtils.mergeTemplateIntoString("price: ${price} tax: ${tax}",model);
        assertEquals("price: 100 tax: 22",parsedString);

        String parsedStringWithMath = VelocityUtils.mergeTemplateIntoString("price: $number.integer($math.div($price,2))",model);
        assertEquals("price: 50",parsedStringWithMath);

    }
}

would it be better to use SPel instead?

Answer 1

I agree that this is kind of off topic, but I think it merits an answer nonetheless.

The whole idea of using a templating engine is that you need to have access to the templates at runtime. If that is the case, then sure, Velocity is a good choice. Then you could provide new versions of the HTML, and assuming you didn't change the variables that were used, you would not have to provide a new version of the application itself (recompiled).

However, if you are just using Velocity to save yourself time, it's not saving much here: you could do this with the StringTokenizer in only a few more lines of code.