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30.4. Expressions

Overview
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The simple language provides various elementary expressions that return different parts of a message exchange. For example, the expression, simple("${header.timeOfDay}"), would return the contents of a header called timeOfDay from the incoming message.
Note
Since Apache Camel 2.9, you must always use the placeholder syntax, ${Expression}, to return a variable value. It is never permissible to omit the enclosing tokens (${ and }).

Contents of a single variable
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You can use the simple language to define string expressions, based on the variables provided. For example, you can use a variable of the form, in.header.HeaderName, to obtain the value of the HeaderName header, as follows: 
simple("${in.header.foo}")
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Variables embedded in a string
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You can embed simple variables in a string expression—for example: 
simple("Received a message from ${in.header.user} on ${date:in.header.date:yyyyMMdd}.")
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date and bean variables
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As well as providing variables that access all of the different parts of an exchange (see Table 30.1, “Variables for the Simple Language”), the simple language also provides special variables for formatting dates, date:command:pattern, and for calling bean methods, bean:beanRef. For example, you can use the date and the bean variables as follows: 
simple("Todays date is ${date:now:yyyyMMdd}")
simple("The order type is ${bean:orderService?method=getOrderType}")
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Specifying the result type
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You can specify the result type of an expression explicitly. This is mainly useful for converting the result type to a boolean or numerical type.
In the Java DSL, specify the result type as an extra argument to simple(). For example, to return an integer result, you could evaluate a simple expression as follows: 
...
.setHeader("five", simple("5", Integer.class))
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In the XML DSL, specify the result type using the resultType attribute. For example: 
<setHeader headerName="five">
  <!-- use resultType to indicate that the type should be a java.lang.Integer --> 
  <simple resultType="java.lang.Integer">5</simple>
</setHeader>
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Nested expressions
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Simple expressions can be nested—for example: 
simple("${header.${bean:headerChooser?method=whichHeader}}")
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Accessing constants or enums
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You can access a bean's constant or enum fields using the following syntax: 
type:ClassName.Field
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For example, consider the following Java enum type: 
package org.apache.camel.processor;
...
public enum Customer {
    GOLD, SILVER, BRONZE
}
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You can access the Customer enum fields, as follows: 
 from("direct:start")
    .choice()
        .when().simple("${header.customer} ==
          ${type:org.apache.camel.processor.Customer.GOLD}")
            .to("mock:gold")
        .when().simple("${header.customer} ==
          ${type:org.apache.camel.processor.Customer.SILVER}")
            .to("mock:silver")
        .otherwise()
            .to("mock:other");
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OGNL expressions
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The Object Graph Navigation Language (OGNL) is a notation for invoking bean methods in a chain-like fashion. If a message body contains a Java bean, you can easily access its bean properties using OGNL notation. For example, if the message body is a Java object with a getAddress() accessor, you can access the Address object and the Address object's properties as follows: 
simple("${body.address}")
simple("${body.address.street}")
simple("${body.address.zip}")
simple("${body.address.city}")
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Where the notation, ${body.address.street}, is shorthand for ${body.getAddress.getStreet}.

OGNL null-safe operator
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You can use the null-safe operator, ?., to avoid encountering null-pointer exceptions, in case the body does not have an address. For example: 
simple("${body?.address?.street}")
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If the body is a java.util.Map type, you can look up a value in the map with the key, foo, using the following notation: 
simple("${body[foo]?.name}")
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OGNL list element access
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You can also use square brackets notation, [k], to access the elements of a list. For example: 
simple("${body.address.lines[0]}")
simple("${body.address.lines[1]}")
simple("${body.address.lines[2]}")
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The last keyword returns the index of the last element of a list. For example, you can access the second last element of a list, as follows: 
simple("${body.address.lines[last-1]}")
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You can use the size method to query the size of a list, as follows: 
simple("${body.address.lines.size}")
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OGNL array length access
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You can access the length of a Java array through the length method, as follows: 
String[] lines = new String[]{"foo", "bar", "cat"};
exchange.getIn().setBody(lines);

simple("There are ${body.length} lines")
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