In my Spring Boot project, I have two JMS listeners listening to one queue. All messages received from the queue have to be processed in the same way and persisted / updated in the database (Oracle). Currently, I have a synchronized method in a class that is doing the parsing of the messages. As expected, all thread read messages simultaneously, but parsing is done one by one as the method (parseMessage()) is synchronized. What I want is to parse the messages simultaneously and do database operations as well.
How can I solve this?
I don't want to create two different classes with the same code and use #Qualifier to call different classes in each listener, as the code for parsing the message is the same.
The ideal solution, I think, is to do database operations using a new synchronized method in a new class, but parsing the message in a multi-threaded way. So, at a time only one thread can say persist / update. When a thread is not waiting to persist / update, it continues the parsing on its own thread.
Please correct me if I am wrong or if you find the optimal solution. Let me know if any other info is needed.
JMS Controller Class
#RestController
#EnableJms
public class JMSController {
#Autowired
private IParseMapXml iParseMapXml;
#JmsListener(destination = "${app.jms_destinaltion}")
public void receiveMessage1(String recvMsg) {
try {
InputSource is = new InputSource(new StringReader(recvMsg.replaceAll("&", "&")));
Document doc = new SAXReader().read(is);
iParseMapXml.parseMessage(doc);
} catch (Exception e) {
}
}
#JmsListener(destination = "${app.jms_destinaltion}")
public void receiveMessage2(String recvMsg) {
try {
InputSource is = new InputSource(new StringReader(recvMsg.replaceAll("&", "&")));
Document doc = new SAXReader().read(is);
iParseMapXml.parseMessage(doc);
} catch (Exception e) {
}
}
}
Parse XML Interface
public interface IParseMapXml {
public void parseMessage(Document doc);
}
Parsing Implementation
public class ParsingMessageClass implements IParseMapXml{
#Override
#Transactional
synchronized public void parseMessage(Document doc) {
// TODO Auto-generated method stub
....
PROCESS DATA/MESSAGE
....
DO DB OPERATIONS
}
}
I'm trying to understand how threads works in java. This is a simple database request that returns a ResultSet. I'm using JavaFx.
package application;
import java.sql.ResultSet;
import java.sql.SQLException;
import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
public class Controller{
#FXML
private Button getCourseBtn;
#FXML
private TextField courseId;
#FXML
private Label courseCodeLbl;
private ModelController mController;
private void requestCourseName(){
String courseName = "";
Course c = new Course();
c.setCCode(Integer.valueOf(courseId.getText()));
mController = new ModelController(c);
try {
ResultSet rs = mController.<Course>get();
if(rs.next()){
courseCodeLbl.setText(rs.getString(1));
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// return courseName;
}
public void getCourseNameOnClick(){
try {
// courseCodeLbl.setText(requestCourseName());
Thread t = new Thread(new Runnable(){
public void run(){
requestCourseName();
}
}, "Thread A");
t.start();
} catch (NumberFormatException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
This returns an exception:
Exception in thread "Thread A" java.lang.IllegalStateException: Not on FX application thread; currentThread = Thread A
How do I correctly implement threading so that every database request is executed in a second thread instead of the main thread?
I've heard of implementing Runnable but then how do I invoke different methods in run method?
Never worked with threading before but I thought it's time for it.
Threading Rules for JavaFX
There are two basic rules for threads and JavaFX:
Any code that modifies or accesses the state of a node that is part of a scene graph must be executed on the JavaFX application thread. Certain other operations (e.g. creating new Stages) are also bound by this rule.
Any code that may take a long time to run should be executed on a background thread (i.e. not on the FX Application Thread).
The reason for the first rule is that, like most UI toolkits, the framework is written without any synchronization on the state of elements of the scene graph. Adding synchronization incurs a performance cost, and this turns out to be a prohibitive cost for UI toolkits. Thus only one thread can safely access this state. Since the UI thread (FX Application Thread for JavaFX) needs to access this state to render the scene, the FX Application Thread is the only thread on which you can access "live" scene graph state. In JavaFX 8 and later, most methods subject to this rule perform checks and throw runtime exceptions if the rule is violated. (This is in contrast to Swing, where you can write "illegal" code and it may appear to run fine, but is in fact prone to random and unpredictable failure at arbitrary time.) This is the cause of the IllegalStateException you are seeing: you are calling courseCodeLbl.setText(...) from a thread other than the FX Application Thread.
The reason for the second rule is that the FX Application Thread, as well as being responsible for processing user events, is also responsible for rendering the scene. Thus if you perform a long-running operation on that thread, the UI will not be rendered until that operation is complete, and will become unresponsive to user events. While this won't generate exceptions or cause corrupt object state (as violating rule 1 will), it (at best) creates a poor user experience.
Thus if you have a long-running operation (such as accessing a database) that needs to update the UI on completion, the basic plan is to perform the long-running operation in a background thread, returning the results of the operation when it is complete, and then schedule an update to the UI on the UI (FX Application) thread. All single-threaded UI toolkits have a mechanism to do this: in JavaFX you can do so by calling Platform.runLater(Runnable r) to execute r.run() on the FX Application Thread. (In Swing, you can call SwingUtilities.invokeLater(Runnable r) to execute r.run() on the AWT event dispatch thread.) JavaFX (see later in this answer) also provides some higher-level API for managing the communication back to the FX Application Thread.
General Good Practices for Multithreading
The best practice for working with multiple threads is to structure code that is to be executed on a "user-defined" thread as an object that is initialized with some fixed state, has a method to perform the operation, and on completion returns an object representing the result. Using immutable objects, in particular, a record, for the initialized state and computation result is highly desirable. The idea here is to eliminate the possibility of any mutable state being visible from multiple threads as far as possible. Accessing data from a database fits this idiom nicely: you can initialize your "worker" object with the parameters for the database access (search terms, etc). Perform the database query and get a result set, use the result set to populate a collection of domain objects, and return the collection at the end.
In some cases it will be necessary to share mutable state between multiple threads. When this absolutely has to be done, you need to carefully synchronize access to that state to avoid observing the state in an inconsistent state (there are other more subtle issues that need to be addressed, such as liveness of the state, etc). The strong recommendation when this is needed is to use a high-level library to manage these complexities for you.
Using the javafx.concurrent API
JavaFX provides a concurrency API that is designed for executing code in a background thread, with API specifically designed for updating the JavaFX UI on completion of (or during) the execution of that code. This API is designed to interact with the java.util.concurrent API, which provides general facilities for writing multithreaded code (but with no UI hooks). The key class in javafx.concurrent is Task, which represents a single, one-off, unit of work intended to be performed on a background thread. This class defines a single abstract method, call(), which takes no parameters, returns a result, and may throw checked exceptions. Task implements Runnable with its run() method simply invoking call(). Task also has a collection of methods which are guaranteed to update state on the FX Application Thread, such as updateProgress(...), updateMessage(...), etc. It defines some observable properties (e.g. state and value): listeners to these properties will be notified of changes on the FX Application Thread. Finally, there are some convenience methods to register handlers (setOnSucceeded(...), setOnFailed(...), etc); any handlers registered via these methods will also be invoked on the FX Application Thread.
So the general formula for retrieving data from a database is:
Create a Task to handle the call to the database.
Initialize the Task with any state that is needed to perform the database call.
Implement the task's call() method to perform the database call, returning the results of the call.
Register a handler with the task to send the results to the UI when it is complete.
Invoke the task on a background thread.
For database access, I strongly recommend encapsulating the actual database code in a separate class that knows nothing about the UI (Data Access Object design pattern). Then just have the task invoke the methods on the data access object.
So you might have a DAO class like this (note there is no UI code here):
public class WidgetDAO {
// In real life, you might want a connection pool here, though for
// desktop applications a single connection often suffices:
private Connection conn ;
public WidgetDAO() throws Exception {
conn = ... ; // initialize connection (or connection pool...)
}
public List<Widget> getWidgetsByType(String type) throws SQLException {
try (PreparedStatement pstmt = conn.prepareStatement("select * from widget where type = ?")) {
pstmt.setString(1, type);
ResultSet rs = pstmt.executeQuery();
List<Widget> widgets = new ArrayList<>();
while (rs.next()) {
Widget widget = new Widget();
widget.setName(rs.getString("name"));
widget.setNumberOfBigRedButtons(rs.getString("btnCount"));
// ...
widgets.add(widget);
}
return widgets ;
}
}
// ...
public void shutdown() throws Exception {
conn.close();
}
}
Retrieving a bunch of widgets might take a long time, so any calls from a UI class (e.g a controller class) should schedule this on a background thread. A controller class might look like this:
public class MyController {
private WidgetDAO widgetAccessor ;
// java.util.concurrent.Executor typically provides a pool of threads...
private Executor exec ;
#FXML
private TextField widgetTypeSearchField ;
#FXML
private TableView<Widget> widgetTable ;
public void initialize() throws Exception {
widgetAccessor = new WidgetDAO();
// create executor that uses daemon threads:
exec = Executors.newCachedThreadPool(runnable -> {
Thread t = new Thread(runnable);
t.setDaemon(true);
return t ;
});
}
// handle search button:
#FXML
public void searchWidgets() {
final String searchString = widgetTypeSearchField.getText();
Task<List<Widget>> widgetSearchTask = new Task<List<Widget>>() {
#Override
public List<Widget> call() throws Exception {
return widgetAccessor.getWidgetsByType(searchString);
}
};
widgetSearchTask.setOnFailed(e -> {
widgetSearchTask.getException().printStackTrace();
// inform user of error...
});
widgetSearchTask.setOnSucceeded(e ->
// Task.getValue() gives the value returned from call()...
widgetTable.getItems().setAll(widgetSearchTask.getValue()));
// run the task using a thread from the thread pool:
exec.execute(widgetSearchTask);
}
// ...
}
Notice how the call to the (potentially) long-running DAO method is wrapped in a Task which is run on a background thread (via the accessor) to prevent blocking the UI (rule 2 above). The update to the UI (widgetTable.setItems(...)) is actually executed back on the FX Application Thread, using the Task's convenience callback method setOnSucceeded(...) (satisfying rule 1).
In your case, the database access you are performing returns a single result, so you might have a method like
public class MyDAO {
private Connection conn ;
// constructor etc...
public Course getCourseByCode(int code) throws SQLException {
try (PreparedStatement pstmt = conn.prepareStatement("select * from course where c_code = ?")) {
pstmt.setInt(1, code);
ResultSet results = pstmt.executeQuery();
if (results.next()) {
Course course = new Course();
course.setName(results.getString("c_name"));
// etc...
return course ;
} else {
// maybe throw an exception if you want to insist course with given code exists
// or consider using Optional<Course>...
return null ;
}
}
}
// ...
}
And then your controller code would look like
final int courseCode = Integer.valueOf(courseId.getText());
Task<Course> courseTask = new Task<Course>() {
#Override
public Course call() throws Exception {
return myDAO.getCourseByCode(courseCode);
}
};
courseTask.setOnSucceeded(e -> {
Course course = courseTask.getCourse();
if (course != null) {
courseCodeLbl.setText(course.getName());
}
});
exec.execute(courseTask);
The API docs for Task have many more examples, including updating the progress property of the task (useful for progress bars..., etc.
Related
JavaFX - Background Thread for SQL Query
Sample for accessing a local database from JavaFX using concurrent tasks for database operations so that the UI remains responsive.
Exception in thread "Thread A" java.lang.IllegalStateException: Not on FX application thread; currentThread = Thread A
The exception is trying to tell you that you are trying to access JavaFX scene graph outside the JavaFX application thread. But where ??
courseCodeLbl.setText(rs.getString(1)); // <--- The culprit
If I can't do this how do I use a background thread?
The are different approaches which leads to similar solutions.
Wrap you Scene graph element with Platform.runLater
There easier and most simple way is to wrap the above line in Plaform.runLater, such that it gets executed on JavaFX Application thread.
Platform.runLater(() -> courseCodeLbl.setText(rs.getString(1)));
Use Task
The better approach to go with these scenarios is to use Task, which has specialized methods to send back updates. In the following example, I am using updateMessage to update the message. This property is bind to courseCodeLbl textProperty.
Task<Void> task = new Task<Void>() {
#Override
public Void call() {
String courseName = "";
Course c = new Course();
c.setCCode(Integer.valueOf(courseId.getText()));
mController = new ModelController(c);
try {
ResultSet rs = mController.<Course>get();
if(rs.next()) {
// update message property
updateMessage(rs.getString(1));
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null;
}
}
public void getCourseNameOnClick(){
try {
Thread t = new Thread(task);
// To update the label
courseCodeLbl.textProperty.bind(task.messageProperty());
t.setDaemon(true); // Imp! missing in your code
t.start();
} catch (NumberFormatException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
This has nothing to do with database. JavaFx, like pretty much all GUI libraries, requires that you only use the main UI thread to modify the GUI.
You need to pass the data from the database back to the main UI thread. Use Platform.runLater() to schedule a Runnable to be run in the main UI thread.
public void getCourseNameOnClick(){
new Thread(new Runnable(){
public void run(){
String courseName = requestCourseName();
Platform.runLater(new Runnable(){
courseCodeLbl.setText(courseName)
});
}
}, "Thread A").start();
}
Alternatively, you can use Task.
I started a netty4 nio server with multiple business threads for handling long-term businesses
like below
public void start(int listenPort, final ExecutorService ignore)
throws Exception {
...
bossGroup = new NioEventLoopGroup();
ioGroup = new NioEventLoopGroup();
businessGroup = new DefaultEventExecutorGroup(businessThreads);
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, ioGroup).channel(NioServerSocketChannel.class)
.childOption(ChannelOption.TCP_NODELAY,
Boolean.parseBoolean(System.getProperty(
"nfs.rpc.tcp.nodelay", "true")))
.childOption(ChannelOption.SO_REUSEADDR,
Boolean.parseBoolean(System.getProperty(
"nfs.rpc.tcp.reuseaddress", "true")))
.childHandler(new ChannelInitializer<SocketChannel>() {
#Override
public void initChannel(SocketChannel ch)
throws Exception {
ch.pipeline().addLast("decoder",
new Netty4ProtocolDecoder());
ch.pipeline().addLast("encoder",
new Netty4ProtocolEncoder());
ch.pipeline().addLast(businessGroup, "handler",
new Netty4ServerHandler());
}
});
b.bind(listenPort).sync();
LOGGER.warn("Server started,listen at: " + listenPort + ", businessThreads is " + businessThreads);
}
I found that there was only one thread working when the server accepted one connection.
How can I bootstrap a server that can start multiple business threads for only one connection?
Thanks,
Mins
Netty will always use the same thread for one connection. It's by design. If you would like to change this you may be able to implement a custom EventExecutorGroup and pass it in when adding your ChannelHandler to the ChannelPipeline.
Be aware this may result in messed up order of packets.
In my Netty SimpleChannelUpstreamHandler when I receive a message I need to start up a connection to another Netty Server and forward the message on. However, when starting up this second connection I use:
ChannelFuture channelFuture = clientBootstrap.connect(new InetSocketAddress(host, port));
hannelFuture.awaitUninterruptibly();
Which results in the following error:
java.lang.IllegalStateException: await*() in I/O thread causes a dead lock or sudden performance drop. Use addListener() instead or call await*() from a different thread.
at org.jboss.netty.channel.DefaultChannelFuture.checkDeadLock(DefaultChannelFuture.java:314)
at org.jboss.netty.channel.DefaultChannelFuture.awaitUninterruptibly(DefaultChannelFuture.java:226)
at com.my.NettyClient.start(NettyClient.java:204)
....
at com.my.MyChannelUpstreamHandler.messageReceived(MyChannelUpstreamHandler.java:52)
Whats the best way to start this second connection? Should I do the following?:
#Override
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
ExecutorService executorService = Executors.newSingleThreadExecutor();
executorService.submit(new Runnable() {
#Override
public void run() {
// Connect to another Netty Server...
// Forward on message...
}
});
executorService.shutdown();
...
Is this wasteful to start a new thread on each message recieved?
Checkout the proxy example to see how you can do it without blocking:
http://netty.io/docs/stable/xref/org/jboss/netty/example/proxy/HexDumpProxyInboundHandler.html
I am developing a Java-ME Based Mobile Application. Now My Requirements are like whenever I am updating one of my RMS, I want my application to be stay in a Freeze kind of mode; which means no other action like clicking button or anything else should happen. My Method is already "Synchronized".
Kindly guide me regarding this question.
Thanks.
The best way to handle this is to "serialize" your tasks. You can do this with a message queue - a class that maintains a Vector of message objects (tasks) and runs code based on each message. The queue runs on a thread that processes each task (message) in series. You create a simple message class for the different tasks - read RMS etc. A message can be an Integer if you like that wraps a number. The operation of adding and retrieving messages is synchronized but the code than does the tasks is not and runs on a simple switch block. The benefit of serializing your tasks is you don't have to worry about concurrency. Here is some of the essential code from a class I use to do this.
class MessageQueue implements Runnable{
Vector messages;
Thread myThread;
volatile boolean stop;
public void start() {
stop=false;
myThread=new Thread(this);
myThread.start();
}
// add message to queue - this is public
public synchronized void addMessage(Message m) {
messages.addElement(m);
if(stop) {
start();
} else {
// wake the thread
notify();
}
}
// get next message from queue - used by this thread
private synchronized Message nextMessage() {
if(stop) return null;
if(messages.isEmpty()) {
return null;
} else {
Message m=(Message)messages.firstElement();
messages.removeElementAt(0);
return m;
}
}
public void run() {
while (!stop) {
// make thread wait for messages
if (messages.size() == 0) {
synchronized (this) {
try {
wait();
} catch (Exception e) {
}
}
}
if (stop) {
// catch a call to quit
return;
}
processMessage();
}
}
}
// all the tasks are in here
private void processMessage() {
Message m = nextMessage();
switch (m.getType()) {
case Message.TASK1:
// do stuff
break;
case Message.TASK2:
// do other stuff
break;
case Message.TASK3:
// do other other stuff
break;
default: //handle bad message
}
}
}
What you are asking is very code depended. Usually when you want to make some synchronic actions you just write them one after the other. in java it's more complected, since sometimes you "ask" the system to do something (like repaint() method). But since the RMS read/write operations are very quick (few millisecond) i don't see any need in freesing.
Could you please provide some more information about the need (time for RMS to respond)? does your code runs on system thread (main thread) or your own thread?
I want my application to be stay in a Freeze kind of mode; which means no other action like clicking button or anything else should happen.
First of all I would strongly advise against real freezing of UI - this could make a suicidal user experience for your application.
If you ever happened to sit in front of computer frozen because of some programming bug, you may understand why approach like this is strongly discouraged. As they describe it in MIDP threading tutorial, "user interface freezes, the device appears to be dead, and the user becomes frustrated..."
This tutorial by the way also suggests possibly the simplest solution for problems like you describe: displaying a wait screen. If you don't really have reasons to avoid this solution, just do as tutorial suggests.
To be on a safe side, consider serializing tasks as suggested in another answer. This will ensure that when RMS update starts, there are no other tasks pending.