Java Threads

Internal

• java

Subjects

• java Thread States
• java ThreadLocal

Concepts

Daemon Thread

http://www.javaperspective.com/daemon-threads.html

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:

Non-Daemon Threads and "Natural" Shutdown

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

OutOfMemoryError "unable to create new native thread"

A simple Java program that allows experimenting with threads:

https://github.com/NovaOrdis/playground/tree/master/java/threads/multiple-threads-spinning

Java Thread Stack Memory Management

Thread Stack Memory Management

Difference between Blocked and Waiting

http://geekexplains.blogspot.com/2008/07/threadstate-in-java-blocked-vs-waiting.html

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.