Java Agent

JaCoCo uses class file instrumentation to record execution coverage data. Class files are instrumented on-the-fly using a so called Java agent. This mechanism allows in-memory pre-processing of all class files during class loading independent of the application framework.

If you use the JaCoCo Ant tasks or JaCoCo Maven plug-in you don't have to care about the agent and its options directly. This is transparently handled by the them.

The JaCoCo agent collects execution information and dumps it on request or when the JVM exits. There are three different modes for execution data ouput:

The agent jacocoagent.jar is part of the JaCoCo distribution and includes all required dependencies. A Java agent can be activated with the following JVM option:

  -javaagent:[yourpath/]jacocoagent.jar=[option1]=[value1],[option2]=[value2]

The JaCoCo agent accepts the following options:

Option Description Default
destfile Path to the output file for execution data. jacoco.exec
append If set to true and the execution data file already exists, coverage data is appended to the existing file. If set to false, an existing execution data file will be replaced. true
includes A list of class names that should be included in execution analysis. The list entries are separated by a colon (:) and may use wildcard characters (* and ?). Except for performance optimization or technical corner cases this option is normally not required. * (all classes)
excludes A list of class names that should be excluded from execution analysis. The list entries are separated by a colon (:) and may use wildcard characters (* and ?). Except for performance optimization or technical corner cases this option is normally not required. empty (no excluded classes)
exclclassloader A list of class loader names that should be excluded from execution analysis. The list entries are separated by a colon (:) and may use wildcard characters (* and ?). This option might be required in case of special frameworks that conflict with JaCoCo code instrumentation, in particular class loaders that do not have access to the Java runtime classes. sun.reflect.DelegatingClassLoader
sessionid A session identifier that is written with the execution data. Without this parameter a random identifier is created by the agent. auto-generated
dumponexit If set to true coverage data will be written on VM shutdown. The dump can only be written if either file is specified or the output is tcpserver/tcpclient and a connection is open at the time when the VM terminates. true
output Output method to use for writing coverage data. Valid options are:
  • file: At VM termination execution data is written to the file specified in the destfile attribute.
  • tcpserver: The agent listens for incoming connections on the TCP port specified by the address and port attribute. Execution data is written to this TCP connection.
  • tcpclient: At startup the agent connects to the TCP port specified by the address and port attribute. Execution data is written to this TCP connection.
  • none: Do not produce any output.
Please see the security considerations below.
file
address IP address or hostname to bind to when the output method is tcpserver or connect to when the output method is tcpclient. In tcpserver mode the value "*" causes the agent to accept connections on any local address. loopback interface
port Port to bind to when the output method is tcpserver or connect to when the output method is tcpclient. In tcpserver mode the port must be available, which means that if multiple JaCoCo agents should run on the same machine, different ports have to be specified. 6300
classdumpdir Location relative to the working directory where all class files seen by the agent are dumped to. This can be useful for debugging purposes or in case of dynamically created classes for example when scripting engines are used. no dumps
jmx If set to true the agent exposes functionality via JMX under the name org.jacoco:type=Runtime. Please see the security considerations below. false

Security Consideration for Remote Agent Control

The ports and connections opened in tcpserver and tcpclient mode and the JMX interface do not provide any authentication mechanism. If you run JaCoCo on production systems make sure that no untrusted sources have access to the TCP server port, or JaCoCo TCP clients only connect to trusted targets. Otherwise internal information of the application might be revealed or DOS attacks are possible.