Every thread has a priority. Threads with higher priority are executed in preference to threads with lower priority. Each thread may or may not also be marked as a daemon. When code running in some thread creates a new Thread object, the new thread has its priority initially set equal to the priority of the creating thread, and is a daemon thread if and only if the creating thread is a daemon.

When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class). The Java Virtual Machine continues to execute threads until either of the following occurs:

• The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
• All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method.

There are two ways to create a new thread of execution. One is to declare a class to be a subclass of Thread. This subclass should override the run method of class Thread. An instance of the subclass can then be allocated and started. For example, a thread that computes primes larger than a stated value could be written as follows:

 
class PrimeThread extends Thread { 
long minPrime; 
PrimeThread(long minPrime) { 
this.minPrime = minPrime; 
} 

public void run() { 
// compute primes larger than minPrime 
 . . . 
} 
} 

The following code would then create a thread and start it running:

 
PrimeThread p = new PrimeThread(143); 
p.start(); 

The other way to create a thread is to declare a class that implements the Runnable interface. That class then implements the run method. An instance of the class can then be allocated, passed as an argument when creating Thread, and started. The same example in this other style looks like the following:

 
class PrimeRun implements Runnable { 
long minPrime; 
PrimeRun(long minPrime) { 
this.minPrime = minPrime; 
} 

public void run() { 
// compute primes larger than minPrime 
 . . . 
} 
} 

The following code would then create a thread and start it running:

 
PrimeRun p = new PrimeRun(143); 
new Thread(p).start(); 

Every thread has a name for identification purposes. More than one thread may have the same name. If a name is not specified when a thread is created, a new name is generated for it.

If group is null and there is a security manager, the group is determined by the security manager's getThreadGroup method. If group is null and there is not a security manager, or the security manager's getThreadGroup method returns null, the group is set to be the same ThreadGroup as the thread that is creating the new thread.

If there is a security manager, its checkAccess method is called with the ThreadGroup as its argument. This may result in a SecurityException.

If the target argument is not null, the run method of the target is called when this thread is started. If the target argument is null, this thread's run method is called when this thread is started.

The priority of the newly created thread is set equal to the priority of the thread creating it, that is, the currently running thread. The method setPriority may be used to change the priority to a new value.

The newly created thread is initially marked as being a daemon thread if and only if the thread creating it is currently marked as a daemon thread. The method setDaemon may be used to change whether or not a thread is a daemon.


Thread
public Thread( ThreadGroup group, Runnable target, String name, long stackSize )

Allocates a new Thread object so that it has target as its run object, has the specified name as its name, belongs to the thread group referred to by group, and has the specified stack size.

This constructor is identical to Thread(ThreadGroup,Runnable,String) with the exception of the fact that it allows the thread stack size to be specified. The stack size is the approximate number of bytes of address space that the virtual machine is to allocate for this thread's stack. The effect of the stackSize parameter, if any, is highly platform dependent.

On some platforms, specifying a higher value for the stackSize parameter may allow a thread to achieve greater recursion depth before throwing a StackOverflowError. Similarly, specifying a lower value may allow a greater number of threads to exist concurrently without throwing an an OutOfMemoryError (or other internal error). The details of the relationship between the value of the stackSize parameter and the maximum recursion depth and concurrency level are platform-dependent. On some platforms, the value of the stackSize parameter may have no effect whatsoever.

The virtual machine is free to treat the stackSize parameter as a suggestion. If the specified value is unreasonably low for the platform, the virtual machine may instead use some platform-specific minimum value; if the specified value is unreasonably high, the virtual machine may instead use some platform-specific maximum. Likewise, the virtual machine is free to round the specified value up or down as it sees fit (or to ignore it completely).

Specifying a value of zero for the stackSize parameter will cause this constructor to behave exactly like the Thread(ThreadGroup, Runnable, String) constructor.

Due to the platform-dependent nature of the behavior of this constructor, extreme care should be exercised in its use. The thread stack size necessary to perform a given computation will likely vary from one JRE implementation to another. In light of this variation, careful tuning of the stack size parameter may be required, and the tuning may need to be repeated for each JRE implementation on which an application is to run.

Implementation note: Java platform implementers are encouraged to document their implementation's behavior with respect to the stackSize parameter.


Thread
public Thread( ThreadGroup group, String name )

Allocates a new Thread object. This constructor has the same effect as Thread(group, null, name)

First, if there is a security manager, that enumerate method calls the security manager's checkAccess method with the thread group as its argument. This may result in throwing a SecurityException.


holdsLock
public static native boolean holdsLock( Object obj )

Returns true if and only if the current thread holds the monitor lock on the specified object.

This method is designed to allow a program to assert that the current thread already holds a specified lock:

 
assert Thread.holdsLock(obj); 

If there is a security manager, its checkAccess method is called with this thread as its argument. This may result in throwing a SecurityException.

Note: This method was mistakenly non-final in JDK 1.1. It has been made final in the Java 2 Platform.


countStackFrames
public native int countStackFrames( )

Counts the number of stack frames in this thread. The thread must be suspended.


destroy
public void destroy( )

Destroys this thread, without any cleanup. Any monitors it has locked remain locked. (This method is not implemented.)


getContextClassLoader
public ClassLoader getContextClassLoader( )

Returns the context ClassLoader for this Thread. The context ClassLoader is provided by the creator of the thread for use by code running in this thread when loading classes and resources. If not set, the default is the ClassLoader context of the parent Thread. The context ClassLoader of the primordial thread is typically set to the class loader used to load the application.

First, if there is a security manager, and the caller's class loader is not null and the caller's class loader is not the same as or an ancestor of the context class loader for the thread whose context class loader is being requested, then the security manager's checkPermission method is called with a RuntimePermission("getClassLoader") permission to see if it's ok to get the context ClassLoader..


getName
public final String getName( )

Returns this thread's name.


getPriority
public final int getPriority( )

Returns this thread's priority.


getThreadGroup
public final ThreadGroup getThreadGroup( )

Returns the thread group to which this thread belongs. This method returns null if this thread has died (been stopped).


interrupt
public void interrupt( )

Interrupts this thread.

First the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown.

If this thread is blocked in an invocation of the wait(), wait(long), or wait(long,int) methods of the Object class, or of the join(), join(long), join(long,int), sleep(long), or sleep(long,int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.

If this thread is blocked in an I/O operation upon an channel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a java.nio.channels.ClosedByInterruptException.

If this thread is blocked in a java.nio.channels.Selector then the thread's interrupt status will be set and it will return immediately from the selection operation, possibly with a non-zero value, just as if the selector's wakeup method were invoked.

If none of the previous conditions hold then this thread's interrupt status will be set.