The JVM Tool Interface (JVMTI) contains extensive facilities to find out about the internals of a running JVM, including facilities to monitor and modify a running Java program. These facilities are rather low level and require the use of the Java Native Interface (JNI) and C Language programming. However, they provide the opportunity to access fields that would not normally be accessible. Also, there are facilities that can change the behavior of a running Java program (for example, threads can be suspended or stopped). Its profiling tools also allow measuring time that a thread takes to execute, leaving it vulnerable to timing attacks.
The JVMTI works by using agents that communicate with the running JVM. These agents are usually loaded at JVM startup via one of the command line options -agentlib: or -agentpath:. However, some JVMs allow agents to be started when the JVM is actually running. Also, platforms that support environment variables allow agents to be specified in such variables, although this feature can be disabled where security is a concern. The JVMTI is always enabled, and JVMTI agents may run under the default security manager without requiring any permissions to be granted.
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