Home > Articles > Networking > Network Design & Architecture

  • Print
  • + Share This

7.2 The DataLogger Class

We'll start by exploring the main class of the application: DataLogger. DataLogger implements the network (TCP/IP) server and accepts and manages inbound connections from remote clients. A skeleton of the DataLogger class including its constructor is shown in Listing 7.1. The DataLogger class extends Thread and overrides the run method, making it the server's main loop. The primordial thread is allowed to die after successful initialization of the application.

During construction of DataLogger an instance of HumidityLogger is created specifying the sample count and delay time in seconds between samples. After the logger thread is started, the DataLogger thread is not concerned with the operation of the logger or even what kind of data it is collecting. It maintains a reference to the logger object that is used to satisfy log requests for inbound client connections. We'll cover the data collection classes in detail in the next section.

Listing 7.1 DataLogger

import java.io.*;
import java.net.*;

class DataLogger extends Thread {
  static final int SERVER_PORT = 5588;

  HumidityLogger logger;

  DataLogger(int samples, int delay) throws LoggingException {
    // Create and start the logging daemon
    logger = new HumidityLogger(samples, delay);


DataLogger requires the number of data readings to be maintained and the delay in seconds between each reading to be specified on the command line. DataLogger's main method is shown in Listing 7.2.

Listing 7.2 DataLogger's main method

public static void main(String[] args) {
  System.out.println("Starting DataLogger ...");
  if (args.length != 2) {
    System.out.println("Usage: java DataLogger samples delay");
  int samples = Integer.parseInt(args[0]);
  int delay = Integer.parseInt(args[1]);
  try {
    (new DataLogger(samples, delay)).start();
  } catch (Exception e) {
    System.out.println("Error creating data logger");
    // In case any non-daemon threads have been started
    // System.exit(1);

After extracting the samples and count values from the command line, the main method creates a new DataLogger object, which also creates a new thread of execution. After constructing the new instance of DataLogger, the start method is invoked to kick off the server.

The server spends eternity in the run method, processing network connections. DataLogger's run method along with the inner class LogWorker is shown in Listing 7.3. It starts by creating a ServerSocket object to listen for inbound connections from remote clients. The SERVER_PORT number used in creating the ServerSocket object is simply chosen as a large magic number. Anything that is comfortably above the range of "well-known port"2 numbers will do. As implemented here, DataLogger uses a port number of 5588. The application could easily be modified to use a port number specified on the command line.

Listing 7.3 DataLogger's run method

public void run() {
  ServerSocket ss = null;
  try {
    ss = new ServerSocket(SERVER_PORT);    
  } catch (Exception e) {
    // Abort if we can't create ServerSocket instance

  while (true) {
    Socket s = null;
    try {  
      // Wait for client connections over PPP or Ethernet
      s = ss.accept();
    } catch (IOException ioe) {
      // Shut down the logging daemon
      System.out.println("Fatal problem with server socket");
      // Fall out of run method

    // Create a new thread to handle this connection
    (new LogWorker(s)).start();

private class LogWorker extends Thread {
  private Socket s;

  private LogWorker(Socket s) {
    this.s = s;

  public void run() {
    DataOutputStream dout = null;
    try {
      dout = new DataOutputStream(
            new BufferedOutputStream(s.getOutputStream()));
    } catch (IOException ioe) {
      System.out.println("I/O error writing log data"); 
    } finally {
      try {
      } catch (IOException e) {}

After the ServerSocket object is created, the run method enters an infinite loop that accepts and processes inbound client connections. After a new instance of Socket is returned from the ServerSocket object's accept method, a new thread (an instance of LogWorker) is created to manage the connection. The socket's getOutputStream method is invoked to obtain the lowest-level output stream (an instance of SocketOutputStream) for writing data to the underlying connection. This stream is used in constructing an instance of BufferedOutputStream, and the resulting buffered output stream is wrapped in a DataOutputStream.

The idea of using a buffered DataOutputStream for writing the log data is that the data-collecting daemon will write all of the samples it has collected to the output stream in an iterative fashion. If the output stream were not buffered, every write method invoked on the output stream would perform a write to the low-level SocketOutputStream. This forces a write to the native socket layer. Writing the log data in such a fashion could be termed "byte-banging." Byte-banging is very inefficient, since each of these writes is fairly expensive. With the BufferedOutputStream, writes to the underlying SocketOutputStream occur only when the BufferedOutputStreams internal buffer is full or the stream's flush method is invoked. The default internal buffer size used in TINI's implementation of all buffered streams is 512 bytes. This allows several log entries to be written to the DataOutputStream before the write method on the SocketOutputStream is invoked to write the contents of the buffer to the native socket layer.

The DataOutputStream object is passed to the writeLog method of the data collecting daemon. The writeLog method is responsible for writing all data points to the output stream. After the writeLog method returns, any data remaining in the output stream's internal buffer is flushed and both the stream and underlying socket are closed.

There are two catch blocks in the run method. The first protects the accept method. If an IOException is thrown from accept, the problem is assumed to be fatal. There isn't any good reason for accept to throw a runtime exception other than that the port selected is already owned by another thread or process and this problem won't be fixed with retries. In this case, the while loop is exited by the break statement, allowing the DataLogger thread to exit. As we'll see in the next few sections, all other threads created in the DataLogger process are daemon threads, so when the server thread exits, all of the other threads stop executing and the application terminates. The other catch block protects the writing of the log data to the remote client. In this case, an error could result from the client terminating the connection unexpectedly. While this certainly does prohibit the successful transfer of the log data, it shouldn't cause the application to exit. In this case, we just close down the socket and output stream and wait for a new connection.

In this section we developed the top-level framework necessary to accept network connections and dispatch output requests to the data collector. Next, we'll focus on the details of collecting and managing the data samples.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020