Java Threads
Internal
Subjects
Concepts
Daemon Thread
A daemon thread is a thread that DOES NOT prevent the JVM from exiting, even if the thread is in WAITING or TIMED_WAITING state. Specifically the JVM will exit when only daemon threads remain.
When the JVM halts any remaining daemon threads are abandoned: finally blocks are not executed, stacks are not unwound - JVM just exits. Due to this reason daemon threads should be used sparingly and it is dangerous to use them for tasks that might perform any sort of I/O.
When a new thread is created it inherits the daemon status of its parent. Or it can be set with setDaemon(true).
For more details on the relationship between non-daemon threads and JVM shutdown, see:
Java Threads and Linux Processes
On a Linux system, each JVM thread counts as a "process", in that it is subject to system-wide and user-wide process number limitations - see "maximum number of processes allowed on a system" and "maximum number of processes available to a single user". When a JVM instance adjust its number of threads, the change is reflected by /proc/stat "processes" count.
The number of Java threads for a JVM is reported by /proc/<pid>/status.
For an example of what happens when the number of Linux processes allowed to an user is reached, see
A simple Java program that allows experimenting with threads:
Java Thread Stack Memory Management
Difference between Blocked and Waiting
Thread Dump
A thread dump is the result of one invocation of the jstack command, or of sending SIGQUIT (-3) into the Java virtual machine process. A thread dump consists in a set of stack traces, one stack trace for each thread in the Java virtual machine.