Chapter 30. The Simple Language

In the Java DSL, there are two styles for using the simple() command in a route. You can either pass the simple() command as an argument to a processor, as follows:

from("seda:order")
  .filter(simple("${in.header.foo}"))
  .to("mock:fooOrders");

Or you can call the simple() command as a sub-clause on the processor, for example:

from("seda:order")
  .filter()
  .simple("${in.header.foo}")
  .to("mock:fooOrders");

If you are embedding a simple expression inside a plain text string, you must use the placeholder syntax, ${Expression}. For example, to embed the in.header.name expression in a string:

simple("Hello ${in.header.name}, how are you?")

From Java, you can customize the start and end tokens ({ and }, by default) by calling the changeFunctionStartToken static method and the changeFunctionEndToken static method on the SimpleLanguage object.

For example, you can change the start and end tokens to [ and ] in Java, as follows:

// Java
import org.apache.camel.language.simple.SimpleLanguage;
...
SimpleLanguage.changeFunctionStartToken("[");
SimpleLanguage.changeFunctionEndToken("]");

In the XML DSL, you can use a simple expression by putting the expression inside a simple element. For example, to define a route that performs filtering based on the contents of the foo header:

<route id="simpleExample">
  <from uri="seda:orders"/>
  <filter>
    <simple>${in.header.foo}</simple>
    <to uri="mock:fooOrders"/>
  </filter>
</route>

Sometimes — for example, if you have enabled Spring property placeholders or OSGi blueprint property placeholders — you might find that the ${Expression} syntax clashes with another property placeholder syntax. In this case, you can disambiguate the placeholder using the alternative syntax, $simple{Expression}, for the simple expression. For example:

<simple>Hello $simple{in.header.name}, how are you?</simple>

From XML configuration, you can customize the start and end tokens ({ and }, by default) by overriding the SimpleLanguage instance. For example, to change the start and end tokens to [ and ], define a new SimpleLanguage bean in your XML configuration file, as follows:

<bean id="simple" class="org.apache.camel.language.simple.SimpleLanguage">
  <constructor-arg name="functionStartToken" value="["/>
  <constructor-arg name="functionEndToken" value="]"/>
</bean>

By default, whitespace preceding and following a simple expression is automatically trimmed in XML DSL. So this expression with surrounding whitespace:

<transform>
  <simple>
    data=${body}
  </simple>
</transform>

is automatically trimmed, so that it is equivalent to this expression (no surrounding whitespace):

<transform>
  <simple>data=${body}</simple>
</transform>

If you want to include newlines before or after the expression, you can either explicitly add a newline character, as follows:

<transform>
  <simple>data=${body}\n</simple>
</transform>

or you can switch off auto-trimming, by setting the trim attribute to false, as follows:

<transform trim="false">
  <simple>data=${body}
</simple>
</transform>

It is possible to execute Simple scripts that are stored in an external resource, as described here.

Use the following syntax to access a Simple script that is stored as an external resource:

resource:Scheme:Location

Where the Scheme: can be either classpath:, file:, or http:.

For example, to read the mysimple.txt script from the classpath,

simple("resource:classpath:mysimple.txt")

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.

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}")

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}.")

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}")

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))

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>

From Camel 2.17, the setHeaderand setExchange properties allows to use a dynamic header key using the Simple language, if the name of the key is a Simple language expression.

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
      <setHeader headerName="$simple{type:org.apache.camel.spring.processor.SpringSetPropertyNameDynamicTest$TestConstans.EXCHANGE_PROP_TX_FAILED}">
        <simple>${type:java.lang.Boolean.TRUE}</simple>
      </setHeader>
    <to uri="mock:end"/>
  </route>
</camelContext>

Simple expressions can be nested — for example:

simple("${header.${bean:headerChooser?method=whichHeader}}")

You can access a bean’s constant or enum fields using the following syntax:

type:ClassName.Field

For example, consider the following Java enum type:

package org.apache.camel.processor;
...
public enum Customer {
    GOLD, SILVER, BRONZE
}

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");

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}")

Where the notation, ${body.address.street}, is shorthand for ${body.getAddress.getStreet}.

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}")

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}")

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]}")

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]}")

You can use the size method to query the size of a list, as follows:

simple("${body.address.lines.size}")

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")

You can construct predicates by testing expressions for equality. For example, the predicate, simple("${header.timeOfDay} == '14:30'"), tests whether the timeOfDay header in the incoming message is equal to 14:30.

In addition, whenever the resultType is specified as a Boolean the expression is evaluated as a predicate instead of an expression. This allows the predicate syntax to be used for these expressions.

You can also test various parts of an exchange (headers, message body, and so on) using simple predicates. Simple predicates have the following general syntax:

${LHSVariable} Op RHSValue

Where the variable on the left hand side, LHSVariable, is one of the variables shown in Table 30.1, “Variables for the Simple Language” and the value on the right hand side, RHSValue, is one of the following:

The simple language always attempts to convert the RHS value to the type of the LHS value.

For example, you can perform simple string comparisons and numerical comparisons as follows:

simple("${in.header.user} == 'john'")

simple("${in.header.number} > '100'")  // String literal can be converted to integer

You can test whether the left hand side is a member of a comma-separated list, as follows:

simple("${in.header.type} in 'gold,silver'")

You can test whether the left hand side matches a regular expression, as follows:

simple("${in.header.number} regex '\d{4}'")

You can test the type of the left hand side using the is operator, as follows:

simple("${in.header.type} is 'java.lang.String'")
simple("${in.header.type} is 'String'") // You can abbreviate java.lang. types

You can test whether the left hand side lies in a specified numerical range (where the range is inclusive), as follows:

simple("${in.header.number} range '100..199'")

You can also combine predicates using the logical conjunctions, && and ||.

For example, here is an expression using the && conjunction (logical and):

simple("${in.header.title} contains 'Camel' && ${in.header.type} == 'gold'")

And here is an expression using the || conjunction (logical inclusive or):

simple("${in.header.title} contains 'Camel' || ${in.header.type} == 'gold'")

Table 30.1, “Variables for the Simple Language” shows all of the variables supported by the simple language.

The binary operators for simple language predicates are shown in Table 30.2, “Binary Operators for the Simple Language”.

The binary operators for simple language predicates are shown in Table 30.3, “Unary Operators for the Simple Language”.

The conjunctions shown in Table 30.4, “Conjunctions for Simple Language Predicates” can be used to combine two or more simple language predicates.