"Execute Immediate" in Java

PL/SQL中可以用"Execute Immediate"来动态执行SQL或者PL/SQL代码，在有些时候用起来很方便。比如说我要动态创建一个Package，我可以用Excecute Immediate来执行"create or replace package xxx” 来创建一个package, 然后甚至可以执行package里面定义的procedure/function。

Java6中提供的动态编译Java类的功能，然后再借助反射机制，可以很容易实现上面说的类似PL/SQL中的Execute Immediate的功能。比如说如下这个例子...

package com.moodys.fang.play;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URI;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLConnection;
import java.util.Arrays;
import java.util.List;

import javax.tools.JavaCompiler;
import javax.tools.SimpleJavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import javax.tools.JavaCompiler.CompilationTask;

/**
 * @author YuFa
 *
 */
public class MyCalculator {
    
    static class StringSourceJavaObject extends SimpleJavaFileObject { 
        
        private String content = null; 
        public StringSourceJavaObject(String name, String content) throws URISyntaxException { 
           super(URI.create("string:///" + name.replace('.','/') +  Kind.SOURCE.extension), Kind.SOURCE); 
           this.content = content; 
        } 
   
        @Override
        public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOException { 
           return content; 
        } 
     } 
    
    static class MyClassLoader extends ClassLoader {

        public Class loadClass(final String className, final String classFileURL) throws ClassNotFoundException {
            
            try {
                URL classUrl = new URL(classFileURL);
                
                URLConnection connection = classUrl.openConnection();
                InputStream input = connection.getInputStream();
                ByteArrayOutputStream buffer = new ByteArrayOutputStream();
                int data = input.read();
                
                while(data != -1) {
                    buffer.write(data);
                    data = input.read();
                }

                input.close();

                byte[] classData = buffer.toByteArray();
                
                return defineClass(className, classData, 0, classData.length);
                
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace(); 
            }
            
            return null;
        }
    }
    
    
    private static boolean compileSource(final String className, final String source) throws URISyntaxException, IOException {
        
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager fileManager =  compiler.getStandardFileManager(null, null, null); 
        StringSourceJavaObject sourceObject = new MyCalculator.StringSourceJavaObject(className, source); 
        List<StringSourceJavaObject> fileObjects =  Arrays.asList(sourceObject); 
        CompilationTask task = compiler.getTask(null, fileManager, null, null, null, fileObjects); 
        return task.call(); 
    }
    
    public static double calculate(final String expr) throws Exception { 
        final String className = "CalculatorMain"; 
        final String methodName = "calculate"; 
        
        String source = "public class " + className  
                            + " { public static double " + methodName + "() {" +
                                    " return " + expr + ";" +
                                "} " +
                          "}"; 
        
        boolean result = compileSource(className, source);
        
        if (result) { 
            MyClassLoader loader = new MyClassLoader();
            Class<?> clazz = loader.loadClass(className, "file:///" + System.getProperty("user.dir").replace('\\', '/') +  "/" + className + ".class");
            Method method = clazz.getMethod(methodName, new Class<?>[] {}); 
            Object value = method.invoke(null, new Object[] {}); 
            return (Double)value; 
         }
        
        throw new Exception("Failed to compile the source code!");
    }
    
    /**
     * @param args
     * @throws Exception 
     */
    public static void main(String[] args) throws Exception{
        
        System.out.println(calculate("(4+3.3)/5"));
    }

}

 
MyCalculator主要是借助动态生成的类来进行算术表达式的计算，这样就不需要写代码去解析表达式了，而是把这些事情全都交给 JVM来搞定。