Spring 4 官方文档学习（五）核心技术之SpEL

package cn.larry.demo;

import org.junit.Test;
import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;

public class TheTest {
    @Test
    public void run() {
        ExpressionParser parser=new SpelExpressionParser();
        String str="‘Hello World‘";
        
        Expression exp = parser.parseExpression(str);
//        Object value = exp.getValue();
        String value = exp.getValue(String.class);
        
        System.out.println(value);
    }
}

// demo for calling method
@Test
public void run1() {
    ExpressionParser parser = new SpelExpressionParser();
    String str = "‘Hello World‘.concat(‘!!!‘)";

    Expression exp = parser.parseExpression(str);
    Object value = exp.getValue();

    System.out.println(value);
}

// demo for accessing property of JavaBean
@Test
public void run2() {
    ExpressionParser parser = new SpelExpressionParser();
    String str = "‘Hello World‘.bytes";

    Expression exp = parser.parseExpression(str);
    byte[] value = exp.getValue(byte[].class);

    System.out.println(value);
}

// demo for accessing nested property of JavaBean
@Test
public void run3() {
    ExpressionParser parser = new SpelExpressionParser();
    String str = "‘Hello World‘.bytes.length";

    Expression exp = parser.parseExpression(str);
    int value = exp.getValue(int.class);

    System.out.println(value);
}

//demo for calling constructor
@Test
public void run4() {
    ExpressionParser parser = new SpelExpressionParser();
    String str = "new String(‘Hello World‘).toUpperCase()";

    Expression exp = parser.parseExpression(str);
    Object value = exp.getValue();

    System.out.println(value);
}

/**
 * The StandardEvaluationContext is relatively expensive to construct and during repeated usage 
 * it builds up cached state that enables subsequent expression evaluations to be performed more quickly. 
 * For this reason it is better to cache and reuse them where possible, rather than construct a new one for each expression evaluation.
 */
@Test
public void run5() {
    GregorianCalendar cal = new GregorianCalendar();
    cal.set(1856, 7, 9);

    Inventor inventor = new Inventor("Nicolas Tesla", cal.getTime(), "serbian");

    SpelExpressionParser parser = new SpelExpressionParser();
    Expression expression = parser.parseExpression("name");

    // 利用目标对象（root object）构造一个context。开销相对较高，建议仅用于不常改变的对象。
    EvaluationContext evaluationContext = new StandardEvaluationContext(inventor);
    
    inventor.setName("Newton"); // 还是会起作用
    
    // 从指定的context中获取本表达式对应的值。
    String value = (String) expression.getValue(evaluationContext);
    System.out.println(value);
}

// 如果root object可能发生改变，不建议使用其创建一个context，因为开销较高。应该如下：
@Test
public void run6() {
    GregorianCalendar cal = new GregorianCalendar();
    cal.set(1856, 7, 9);

    Inventor inventor = new Inventor("Nicolas Tesla--", cal.getTime(), "serbian");

    SpelExpressionParser parser = new SpelExpressionParser();
    Expression expression = parser.parseExpression("name");

    // 直接从目标对象中获取本表达式对应的值。内部还是会创建一个context。
    // the expression evaluation infrastructure creates and manages a default evaluation context internally
    String value = (String) expression.getValue(inventor);  
    System.out.println(value);
    
    inventor.setName("Newton"); // 也起作用了

    value = (String) expression.getValue(inventor);  
    System.out.println(value);
}

// SpEL默认使用Spring core的conversion service。默认支持泛型。
@Test
public void run8() {
    class Simple {
        public List<Boolean> booleanList = new ArrayList<Boolean>();
    }

    Simple simple = new Simple();
    simple.booleanList.add(true);

    StandardEvaluationContext simpleContext = new StandardEvaluationContext(simple);

    SpelExpressionParser parser = new SpelExpressionParser();
    // false is passed in here as a string. SpEL and the conversion service will
    // correctly recognize that it needs to be a Boolean and convert it
    parser.parseExpression("booleanList[0]").setValue(simpleContext, "false");// 不止能获取，还能设置。

    // b will be false
    Boolean b = simple.booleanList.get(0);
    
    System.out.println(b);
}

// demo for parser configuration
@Test
public void run9() {
    class Demo {
        public List<String> list;
    }

    // Turn on:
    // - auto null reference initialization
    // - auto collection growing
    SpelParserConfiguration config = new SpelParserConfiguration(true, true); // 关键，见JavaDoc

    ExpressionParser parser = new SpelExpressionParser(config);

    Expression expression = parser.parseExpression("list[3]");

    Demo demo = new Demo();

    Object o = expression.getValue(demo); // 执行了才会修改原数据
    
    // demo.list will now be a real collection of 4 entries
    // Each entry is a new empty String
    System.out.println(demo.list);
}

<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>

<bean id="person" class="cn.larry.demo.Person">
    <property name="name" value="#{ systemProperties[‘user.name‘] }"/>

    <!-- 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>