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8.14. Loop

Loop
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The loop pattern enables you to process a message multiple times. It is used mainly for testing.
Default mode
Notice by default the loop uses the same exchange throughout the looping. So the result from the previous iteration is used for the next (eg Pipes and Filters). From Camel 2.8 onwards you can enable copy mode instead. See the options table for more details.

Exchange properties
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On each loop iteration, two exchange properties are set, which can optionally be read by any processors included in the loop.
Expand
Property Description
CamelLoopSize Apache Camel 2.0: Total number of loops
CamelLoopIndex Apache Camel 2.0: Index of the current iteration (0 based)

Java DSL examples
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The following examples show how to take a request from the direct:x endpoint and then send the message repeatedly to mock:result. The number of loop iterations is specified either as an argument to loop() or by evaluating an expression at run time, where the expression must evaluate to an int (or else a RuntimeCamelException is thrown).
The following example passes the loop count as a constant: 
from("direct:a").loop(8).to("mock:result");
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The following example evaluates a simple expression to determine the loop count: 
from("direct:b").loop(header("loop")).to("mock:result");
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The following example evaluates an XPath expression to determine the loop count: 
from("direct:c").loop().xpath("/hello/@times").to("mock:result");
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XML configuration example
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You can configure the same routes in Spring XML.
The following example passes the loop count as a constant: 
<route>
  <from uri="direct:a"/>
  <loop>
    <constant>8</constant>
    <to uri="mock:result"/>
  </loop>
</route>
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The following example evaluates a simple expression to determine the loop count: 
<route>
  <from uri="direct:b"/>
  <loop>
    <header>loop</header>
    <to uri="mock:result"/>
  </loop>
</route>
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Using copy mode
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Now suppose we send a message to direct:start endpoint containing the letter A. The output of processing this route will be that, each mock:loop endpoint will receive AB as message. 
from("direct:start")
     // instruct loop to use copy mode, which mean it will use a copy of the input exchange
     // for each loop iteration, instead of keep using the same exchange all over
     .loop(3).copy()
         .transform(body().append("B"))
         .to("mock:loop")
     .end()
     .to("mock:result");
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However if we do not enable copy mode then mock:loop will receive AB, ABB, ABBB messages. 
from("direct:start")
     // by default loop will keep using the same exchange so on the 2nd and 3rd iteration its
     // the same exchange that was previous used that are being looped all over
     .loop(3)
         .transform(body().append("B"))
         .to("mock:loop")
     .end()
     .to("mock:result");
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The equivalent example in XML DSL in copy mode is as follows: 
<route>
   <from uri="direct:start"/>
   <!-- enable copy mode for loop eip -->
   <loop copy="true">
     <constant>3</constant>
     <transform>
       <simple>${body}B</simple>
     </transform>
     <to uri="mock:loop"/>
   </loop>
   <to uri="mock:result"/>
 </route>
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Options
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The loop DSL command supports the following options:
Expand
Name Default Value Description
copy false Camel 2.8: Whether or not copy mode is used. If false then the same Exchange is being used throughout the looping. So the result from the previous iteration will be visible for the next iteration. Instead you can enable copy mode, and then each iteration is restarting with a fresh copy of the input Exchange.

Do While Loop
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You can perform the loop until a condition is met using a do while loop. The condition will either be true or false.
In DSL, the command is LoopDoWhile. The following example will perform the loop until the message body length is 5 characters or less: 
from("direct:start")
    .loopDoWhile(simple("${body.length} <= 5"))
        .to("mock:loop")
        .transform(body().append("A"))
    .end()
    .to("mock:result");
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In XML, the command is loop doWhile. The following example also performs the loop until the message body length is 5 characters or less: 
<route>
  <from uri="direct:start"/>
  <loop doWhile="true">
    <simple>${body.length} <= 5</simple>
    <to uri="mock:loop"/>
    <transform>
      <simple>A${body}</simple>
    </transform>
  </loop>
  <to uri="mock:result"/>
</route>
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