EL表达式取值数据类型

引言：

  需要用el表达式取值的数据，除了对象，最好都是String类型

jsp EL表达式中的int、float、char

在EL表达式计算过程中：

1、所有的整数数字字面量都是Long类型的；

2、所有小数字面量都是Double类型的；

3、""或''声明的是字符串，即''也是字符串，非char；

4、比较时都是equals比较。

例题：

1、

如${1+2147483647} 结果是多少？

如果在java程序里边运行会得到-2147483648，而在jsp el中会得到2147483648。

2、

<%

    Map<Object, Object> map = new HashMap<Object, Object>();

    map.put(new Long(1), 123);

    request.setAttribute("map", map);

    request.setAttribute("a", new Long(1));

%>

${map[1]}  正确

${map[a]} 正确

3、

<%

    Map<Object, Object> map = new HashMap<Object, Object>();

    map.put(new Integer(1), 123);

    request.setAttribute("map", map);

    request.setAttribute("a", new Long(1));

    request.setAttribute("b", new Integer(1));

%>

${map[1]}  错误

${map[a]}  错误

${map}  正确

4、

<%

    Map<Object, Object> map = new HashMap<Object, Object>();

    map.put(1.1, 123); //map.put(1.1d, 123);

    request.setAttribute("map", map);

    request.setAttribute("a", new Double(1.1));

%>

map.a=${map[1.1]}  正确

map.a=${map[a]}     正确

5、

<%

    Map<Object, Object> map = new HashMap<Object, Object>();

    map.put(1.1f, 123); //map.put(new Float(1.1), 123);

    request.setAttribute("map", map);

    request.setAttribute("a", new Double(1.1));

    request.setAttribute("b", new Float(1.1));

%>

map.a=${map[1.1]}  错误

map.a=${map[a]}     错误

map.a=${map}     正确

补充：

在EL表达式规范2.2中，定义了：

■ The value of an IntegerLiteral ranges from Long.MIN_VALUE to
Long.MAX_VALUE
■ The value of a FloatingPointLiteral ranges from Double.MIN_VALUE to
Double.MAX_VALUE

在tomcat7.0.6实现中，jasper.jar（实现了EL2.2规范）：

AstFloatingPoint表示小数，AstInteger表示整数，其定义如下：

Java代码：

[Java] 纯文本查看 复制代码

public final class AstInteger extends SimpleNode  
{  
  private volatile Number number;  
  
  public AstInteger(int id)  
  {  
    super(id);  
  }  
  
  protected Number getInteger()  
  {  
    if (this.number == null) {  
      try {  
        this.number = new Long(this.image);  
      } catch (ArithmeticException e1) {  
        this.number = new BigInteger(this.image);  
      }  
    }  
    return this.number;  
  }  
  
  public Class<?> getType(EvaluationContext ctx)  
    throws ELException  
  {  
    return getInteger().getClass();  
  }  
  
  public Object getValue(EvaluationContext ctx)  
    throws ELException  
  {  
    return getInteger();  
  }  
}  

Java代码：

[Java] 纯文本查看 复制代码

public final class AstFloatingPoint extends SimpleNode  
{  
  private volatile Number number;  
  
  public AstFloatingPoint(int id)  
  {  
    super(id);  
  }  
  
  public Number getFloatingPoint()  
  {  
    if (this.number == null) {  
      try {  
        this.number = new Double(this.image);  
      } catch (ArithmeticException e0) {  
        this.number = new BigDecimal(this.image);  
      }  
    }  
    return this.number;  
  }  
  
  public Object getValue(EvaluationContext ctx)  
    throws ELException  
  {  
    return getFloatingPoint();  
  }  
  
  public Class<?> getType(EvaluationContext ctx)  
    throws ELException  
  {  
    return getFloatingPoint().getClass();  
  }  
}  

+ - * /实现，此处只看+的：

Java代码：

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package org.apache.el.parser;  
  
import javax.el.ELException;  
import org.apache.el.lang.ELArithmetic;  
import org.apache.el.lang.EvaluationContext;  
  
public final class AstPlus extends ArithmeticNode  
{  
  public AstPlus(int id)  
  {  
    super(id);  
  }  
  
  public Object getValue(EvaluationContext ctx)  
    throws ELException  
  {  
    Object obj0 = this.children[0].getValue(ctx);  
    Object obj1 = this.children[1].getValue(ctx);  
    return ELArithmetic.add(obj0, obj1);  
  }  
}  

其委托给ELArithmetic.add：

Java代码：

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public static final DoubleDelegate DOUBLE = new DoubleDelegate();  
  
 public static final LongDelegate LONG = new LongDelegate();  
  
 private static final Long ZERO = Long.valueOf(0L);  
  
 public static final Number add(Object obj0, Object obj1) {  
   if ((obj0 == null) && (obj1 == null))  
     return Long.valueOf(0L);  
   ELArithmetic delegate;  
   if (BIGDECIMAL.matches(obj0, obj1))  
     delegate = BIGDECIMAL;  
   else if (DOUBLE.matches(obj0, obj1))  
     if (BIGINTEGER.matches(obj0, obj1))  
       delegate = BIGDECIMAL;  
     else  
       delegate = DOUBLE;  
   else if (BIGINTEGER.matches(obj0, obj1))  
     delegate = BIGINTEGER;  
   else {  
     delegate = LONG;  
   }  
   Number num0 = delegate.coerce(obj0);  
   Number num1 = delegate.coerce(obj1);  
  
   return delegate.add(num0, num1);  
 }  

此处委托给了各种delegate计算，其+的实现：

Java代码：

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public static final class LongDelegate extends ELArithmetic  
  {  
    protected Number add(Number num0, Number num1)  
    {  
      return Long.valueOf(num0.longValue() + num1.longValue());  
    }  

从这里我们可以看出其实现。

而且其规范中都规定了具体字面量的东西：
1.3 Literals
There are literals for boolean, integer, floating point, string, and null in an eval-
expression.
■ Boolean - true and false
■ Integer - As defined by the IntegerLiteral construct in Section 1.19
■ Floating point - As defined by the FloatingPointLiteral construct in
Section 1.19
■ String - With single and double quotes - " is escaped as \", ' is escaped as \',
and \ is escaped as \\. Quotes only need to be escaped in a string value enclosed
in the same type of quote
■ Null - null
也规定了操作符的运算规则，如+ - *：
Java代码：

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1.3 Literals  
There are literals for boolean, integer, floating point, string, and null in an eval-expression.  
  ■ Boolean - true and false  
  ■ Integer - As defined by the IntegerLiteral construct in Section 1.19  
  ■ Floating point - As defined by the FloatingPointLiteral construct in  
Section 1.19  
  ■ String - With single and double quotes - " is escaped as \", ' is escaped as \',and \ is escaped as \\. Quotes only need to be escaped in a string value enclosed in the same type of quote  
  ■ Null - null  
  ■ If operator is -, return A.subtract(B)  
  ■ If operator is *, return A.multiply(B)  
  ■ If A or B is a Float, Double,or String containing ., e,or E:  
  ■ If A or B is BigInteger, coerce both A and B to BigDecimal and apply operator.  
  ■ Otherwise, coerce both A and B to Double and apply operator  
  ■ If A or B is BigInteger, coerce both to BigInteger and then:  
  ■ If operator is +, return A.add(B)  
  ■ If operator is -, return A.subtract(B)  
  ■ If operator is *, return A.multiply(B)  
  ■ Otherwise coerce both A and B to Long and apply operator  
  ■ If operator results in exception, error  

如Integer型，直接交给前边介绍的[size=1em]IntegerLiteral。