Stream caching

While stream types (like StreamSource, InputStream and Reader) are commonly used in messaging for performance reasons, they also have an important drawback: they can only be read once. In order to be able to work with message content multiple times, the stream needs to be cached.

Streams are cached in memory. However, for large stream messages, you can set spoolEnabled=true and then large message (over 128 KB) will be cached in a temporary file instead. Camel itself will handle deleting the temporary file once the cached stream is no longer necessary.

StreamCache - Affecting the message payload

The StreamCache will affect your payload object as it will replace the Stream payload with a org.apache.camel.StreamCache object. This StreamCache is capable of being re-readable and thus possible to better be routed within Camel using redelivery or Content Based Router or the likes.

In order to determine if a message payload requires caching, then Camel uses the Type Converter functionality, to determine if the message payload type can be converted into an org.apache.camel.StreamCache instance.

All the classes from the Camel release that implements org.apache.camel.StreamCache is NOT intended for end users to create as instances, but they are part of Camels stream-caching functionality.

Configuring Stream Caching

Stream caching is configured using org.apache.camel.spi.StreamCachingStrategy.

The strategy has the following options:

Option Default Description



Whether stream caching is enabled


To filter stream caching of a given set of allowed/denied classes. By default, all classes that are is allowed. Multiple class names can be separated by comma.


To filter stream caching of a given set of allowed/denied classes. By default, all classes that are is allowed. Multiple class names can be separated by comma.



Whether spool to disk is enabled



Base directory where temporary files for spooled streams should be stored. This option supports naming patterns as documented below.



If set, the temporary files are encrypted using the specified cipher transformation (i.e., a valid stream or 8-bit cipher name such as "RC4", "AES/CTR/NoPadding". An empty name "" is treated as null).


128 KB

Size in bytes when the stream should be spooled to disk instead of keeping in memory. Use a value of 0 or negative to disable it all together so streams is always kept in memory regardless of their size.



A percentage (1 to 99) of current used heap memory to use as threshold for spooling streams to disk. The upper bounds is based on heap committed (guaranteed memory the JVM can claim). This can be used to spool to disk when running low on memory.



If spoolUsedHeapMemoryThreshold is in use, then whether the used heap memory upper limit is either Max or Committed.



Whether any or all SpoolRules must return true to determine if the stream should be spooled or not. This can be used as applying AND/OR binary logic to all the rules. By default it’s AND based.



Sets the buffer size to use when allocating in-memory buffers used for in-memory stream caches.



Whether to remove the spool directory when stopping CamelContext.



Whether utilization statistics is enabled. By enabling this you can see these statics for example with JMX.

SpoolDirectory naming pattern

The following patterns is supported:

  • #uuid# = a random UUID

  • #camelId# = the CamelContext id (e.g. the name)

  • #name# = same as #camelId#

  • #counter# = an incrementing counter

  • #bundleId# = the OSGi bundle id (only for OSGi environments)

  • #symbolicName# = the OSGi symbolic name (only for OSGi environments)

  • #version# = the OSGi bundle version (only for OSGi environments)

  • ${env:key} = the environment variable with the key

  • ${key} = the JVM system property with the key

A couple of examples:

To store in the java temp directory with a sub directory using the CamelContext name:


To store in KARAF_HOME/tmp/bundleId directory:


Configuring StreamCachingStrategy in Java

You can configure the StreamCachingStrategy in Java as shown below:

context.getStreamCachingStrategy().setSpoolThreshold(64 * 1024);
context.getStreamCachingStrategy().setBufferSize(16 * 1024);
// to enable encryption using RC4
// context.getStreamCachingStrategy().setSpoolCipher("RC4");

And remember to enable Stream caching on the CamelContext:


or on routes:


Configuring StreamCachingStrategy in XML

In XML you can enable stream caching on the <camelContext> and then do the configuration in the streamCaching element:

<camelContext streamCache="true">

  <streamCaching id="myCacheConfig" bufferSize="16384" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolThreshold="65536"/>

    <from uri="direct:c"/>
    <to uri="mock:c"/>


Using spoolUsedHeapMemoryThreshold

By default, stream caching will spool only big payloads (128 KB or bigger) to disk. However you can also set the spoolUsedHeapMemoryThreshold option which is a percentage of used heap memory. This can be used to also spool to disk when running low on memory.

For example with:

<streamCaching id="myCacheConfig" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolUsedHeapMemoryThreshold="70"/>

Then notice that as spoolThreshold is default enabled with 128 KB, then we have both thresholds in use (spoolThreshold and spoolUsedHeapMemoryThreshold). And in this example then we only spool to disk if payload is > 128 KB and that used heap memory is > 70%. The reason is that we have the option anySpoolRules as default false. That means both rules must be true (e.g. AND).

If we want to spool to disk if either of the rules (e.g. OR), then we can do:

<streamCaching id="myCacheConfig" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolUsedHeapMemoryThreshold="70" anySpoolRules="true"/>

If we only want to spool to disk if we run low on memory then we can set:

<streamCaching id="myCacheConfig" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolThreshold="-1" spoolUsedHeapMemoryThreshold="70"/>

then we do not use the spoolThreshold rule, and only the heap memory based is in use.

By default, the upper limit of the used heap memory is based on the maximum heap size. Though you can also configure to use the committed heap size as the upper limit, this is done using the spoolUsedHeapMemoryLimit option as shown below:

<streamCaching id="myCacheConfig" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolUsedHeapMemoryThreshold="70" spoolUsedHeapMemoryLimit="Committed"/>

Using custom SpoolRule implementations

You can implement your custom rules to determine if the stream should be spooled to disk. This can be done by implementing the interface org.apache.camel.spi.StreamCachingStrategy.SpoolRule which has a single method:

boolean shouldSpoolCache(long length);

The length is the length of the stream. To use the rule then add it to the StreamCachingStrategy as shown below:

SpoolRule mySpoolRule = ...

And from XML you need to define a <bean> with your custom rule:

<bean id="mySpoolRule" class=""/>

<streamCaching id="myCacheConfig" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolRules="mySpoolRule"/>

Using the spoolRules attribute on <streamCaching>. if you have more rules, then separate them by comma.

<streamCaching id="myCacheConfig" spoolEnabled="true" spoolDirectory="/tmp/cachedir" spoolRules="mySpoolRule,myOtherSpoolRule"/>