Spring SpEL表达式的理解

Spring的IOC本质就一个容器，也就是一个对象的工厂，我们通过配置文件注册我们的Bean对象，通过他进行对象的组装与床架。

SpEL表达式就是一种字符串编程,类似于JS里面的EVAL的作用，通过它可以运行字符串内容

特点：算是一种动态的编程在配置文件（xml配置文件或者注解表达式）--------------------------主流的编程都是基于GUI的开发模式（XML开发模式）里面的动态编程

重点：要是通过拼接字符串作为代码运行，SpEL就可以实现，一些灵活的功能

<bean id="numberGuess" class="org.spring.samples.NumberGuess">
  <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>
  <!-- other properties -->
</bean>
<bean id="taxCalculator" class="org.spring.samples.TaxCalculator">
  <property name="defaultLocale" value="#{ systemProperties[‘user.region‘] }"/>
  <!-- other properties -->
</bean>
<bean id="numberGuess" class="org.spring.samples.NumberGuess">
  <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>
  <!-- other properties -->
</bean>
<bean id="shapeGuess" class="org.spring.samples.ShapeGuess">
  <property name="initialShapeSeed" value="#{numberGuess.randomNumber }"/>
  <!-- other properties -->
</bean>

ExpressionParser parser = new SpelExpressionParser();
Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
tesla.setPlaceOfBirth(new PlaceOfBirth("Smiljan"));
StandardEvaluationContext context = new StandardEvaluationContext(tesla);
String city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);
System.out.println(city); // Smiljan
tesla.setPlaceOfBirth(null);
city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);
System.out.println(city); // null - does not throw NullPointerException!!!

否则：我们就只能通过JavaCompiler compiler = ToolProvider.getSystemJavaCompiler(); 来编译字符串生成类

package com.test;
import java.io.IOException;
import java.lang.reflect.Method;
import java.net.URI;
import java.util.Arrays;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
public class CompileString {
    public static void main(String[] args) throws Exception {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        System.out.println(""+ToolProvider.getSystemJavaCompiler());
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(
                null, null, null);
        StringObject so = new StringObject(
                "CalculatorTest",
                "class CalculatorTest {"
                        + " public int multiply(int multiplicand, int multiplier) {"
                        + " System.out.println(multiplicand);"
                        + " System.out.println(multiplier);"
                        + " return multiplicand * multiplier;" + " }" + "}");
        JavaFileObject file = so;
        Iterable files = Arrays.asList(file);
        JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager,
                null, null, null, files);
        Boolean result = task.call();
        System.out.println(result);
        if (result) {
            Class clazz = Class.forName("CalculatorTest");
            Object instance = clazz.newInstance();
            Method m = clazz.getMethod("multiply", new Class[] { int.class,
                    int.class });
            Object[] o = new Object[] { 3, 2 };
            System.out.println(m.invoke(instance, o));
        }
    }
}
class StringObject extends SimpleJavaFileObject {
    private String contents = null;
    public StringObject(String className, String contents) throws Exception {
        super(URI.create("string:///" + className.replace(‘.‘, ‘/‘)
                + Kind.SOURCE.extension), Kind.SOURCE);
        this.contents = contents;
    }
    public CharSequence getCharContent(boolean ignoreEncodingErrors)
            throws IOException {
        return contents;
    }
}