The ThreadGroup class is used in Java to represent a thread group, representing a set of threads, which can be managed by a group of threads and thread groups. Threads can be attributed to a thread group, which can have thread objects, thread groups and threads. Such an organization structure is somewhat similar to that of a tree, as shown in the figure.
All threads created by the user belong to the specified thread group. If no thread group is explicitly specified, the thread belongs to the default thread group (that is, the main thread group). By default, the child thread and the parent thread are in the same thread group.
In addition, only when a thread is created can the thread group it belongs to be specified. Threads cannot change the thread group they belong to in the middle of running, that is to say, once a thread specifies the thread group it belongs to, it cannot be changed.
2. Why use thread groups
1. safety
Threads of the same thread group can modify each other's data. However, if in different thread groups, it is impossible to "cross-thread group" to modify data, which can ensure data security to a certain extent.
2. Batch management
Threads or thread group objects can be managed in batches to organize or control threads or thread group objects effectively.
3. Examples of Thread Group Use
Thread Associated Thread Groups: Level 1 Associations
The so-called first-level association is that the parent object has children, but does not create grandchildren. For example, create a thread group and then assign the created threads to that group, so that these threads can be effectively managed. The code example is as follows:
public class ThreadGroupTest {
public static void main(String[] args) {
ThreadGroup rootThreadGroup = new ThreadGroup("root Thread group");
Thread thread0 = new Thread(rootThreadGroup, new MRunnable(), "thread A");
Thread thread1 = new Thread(rootThreadGroup, new MRunnable(), "thread B");
thread0.start();
thread1.start();
}
}
class MRunnable implements Runnable {
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Thread name: " + Thread.currentThread().getName()
+ ", Thread Group: " + Thread.currentThread().getThreadGroup().getName()) ;
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
//Copy code
The results are as follows:
Thread Name: Thread A, Thread Group: root Thread Group Thread Name: Thread B, Thread Group: root Thread Group Copy code
2. Thread Associated Thread Groups: Multilevel Associations
The so-called multi-level association is that there are children in the parent object, and the creation of grandchildren in the child object will have the effect of descendants. For example, using the second construction method in the following figure, we can assign the sub-thread group to a thread group, and then the created thread to the sub-thread group, which will have the effect of the thread tree.
The code example is as follows:
public class ThreadGroupTest {
public static void main(String[] args) {
ThreadGroup rootThreadGroup = new ThreadGroup("root Thread group");
Thread thread0 = new Thread(rootThreadGroup, new MRunnable(), "thread A");
Thread thread1 = new Thread(rootThreadGroup, new MRunnable(), "thread B");
thread0.start();
thread1.start();
ThreadGroup threadGroup1 = new ThreadGroup(rootThreadGroup, "Sub thread group");
Thread thread2 = new Thread(threadGroup1, new MRunnable(), "thread C");
Thread thread3 = new Thread(threadGroup1, new MRunnable(), "thread D");
thread2.start();
thread3.start();
}
}
class MRunnable implements Runnable {
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Thread name: " + Thread.currentThread().getName()
+ ", Thread Group: " + Thread.currentThread().getThreadGroup().getName()
+ ", Parent thread group: " + Thread.currentThread().getThreadGroup().getParent().getName());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
//Copy code
The results are as follows:
Thread name: thread A, thread group: root thread group, parent thread group: main Thread name: Thread B, where thread group: root thread group, parent thread group: main Thread Name: Thread C, Thread Group: Subthread Group, Parent Thread Group: root Thread Group Thread Name: Thread D, Thread Group: Subthread Group, Parent Thread Group: root Thread Group Copy code
3. Threads in batch management group
Thread groups are naturally used to manage threads in batches, such as interrupting threads in groups in batches. The code example is as follows:
public class ThreadGroupTest {
public static void main(String[] args) {
ThreadGroup rootThreadGroup = new ThreadGroup("root Thread group");
Thread thread0 = new Thread(rootThreadGroup, new MRunnable(), "thread A");
Thread thread1 = new Thread(rootThreadGroup, new MRunnable(), "thread B");
thread0.start();
thread1.start();
ThreadGroup threadGroup1 = new ThreadGroup(rootThreadGroup, "Sub thread group");
Thread thread2 = new Thread(threadGroup1, new MRunnable(), "thread C");
Thread thread3 = new Thread(threadGroup1, new MRunnable(), "thread D");
thread2.start();
thread3.start();
rootThreadGroup.interrupt();
System.out.println("Batch interrupt intra-group threads");
}
}
class MRunnable implements Runnable {
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Thread name: " + Thread.currentThread().getName()
+ ", Thread Group: " + Thread.currentThread().getThreadGroup().getName()
+ ", Parent thread group: " + Thread.currentThread().getThreadGroup().getParent().getName());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
break;
}
}
System.out.println(Thread.currentThread().getName() + "end of execution");
}
}
//Copy code
The results are as follows:
Thread name: thread A, thread group: root thread group, parent thread group: main Thread name: Thread B, where thread group: root thread group, parent thread group: main Thread Name: Thread C, Thread Group: Subthread Group, Parent Thread Group: root Thread Group Thread Name: Thread D, Thread Group: Subthread Group, Parent Thread Group: root Thread Group Batch interrupt intra-group threads Thread A Execution End Thread B Execution End Thread C Execution End Thread D Execution End Copy code
This article is just a brief introduction and demonstration of the ThreadGroup class in Java. You can see the JDK API for more thread group operations.
Reference resources:
https://www.cnblogs.com/xrq730/p/4856072.html
https://www.cnblogs.com/barrywxx/p/9976417.html
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