Chapter 15. HyperSQL Network Listeners (Servers)

Server, WebServer, and Servlet

Fred Toussi

The HSQL Development Group

$Revision: 6692 $

Copyright 2002-2024 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.


Table of Contents

HyperSQL Server
HyperSQL HTTP Server
HyperSQL HTTP Servlet
Server and Web Server Properties
Starting a Server from your Application
Shutting down a Server from your Application
Allowing a Connection to Open or Create a Database
Specifying Database Properties at Server Start
TLS Encryption
Encrypting your JDBC connection
Making a Private-key Keystore
Automatic Server or WebServer startup on UNIX
Network Access Control


As described in the Running and Using HyperSQL chapter, network listeners (servers) provide connectivity to catalogs from different JVM processes. The HyperSQL listeners support both ipv4 and ipv6 network addressing.

HyperSQL Server

This is the preferred way of running a database server and the fastest one. This mode uses the proprietary hsql: communications protocol. The following example of the command for starting the server starts the server with one (default) database with files named "mydb.*" and the public name (alias) of "xdb". Note the database property to set the transaction mode to MVCC is appended to the database file path.

 java -cp ../lib/hsqldb.jar org.hsqldb.server.Server --database.0 file:mydb;hsqldb.tx=mvcc --dbname.0 xdb

Alternatively, a file can be used for passing the arguments to the server. This file must be located in the directory where the command is issued.

 java -cp ../lib/hsqldb.jar org.hsqldb.server.Server

Alternatively, you can specify the path of the file on the command line. In this case, the properties file can have any name or extension, but it should be a valid properties file.

 java -cp ../lib/hsqldb.jar org.hsqldb.server.Server --props myserver.props

Use the --help argument to see the list of available arguments.

 java -cp ../lib/hsqldb.jar org.hsqldb.server.Server --help

The contents of the file is described in the next section.

HyperSQL HTTP Server

This method of access is used when the computer hosting the database server is restricted to the HTTP protocol. The only reason for using this method of access is restrictions imposed by firewalls on the client or server machines and it should not be used where there are no such restrictions. The HyperSQL HTTP Server is a special web server that allows JDBC clients to connect via HTTP. The server can also act as a small general-purpose web server for static pages.

To run an HTTP server, replace the main class for the server in the example command line above with the following:

 java -cp ../lib/hsqldb.jar org.hsqldb.server.WebServer

The contents of the file is described in the next section.

HyperSQL HTTP Servlet

This method of access also uses the HTTP protocol. It is used when a separate servlet engine (or application server) such as Tomcat or Resin provides access to the database. The Servlet Mode cannot be started independently from the servlet engine. The Servlet class, in the HSQLDB jar, should be installed on the application server to provide the connection. The database is specified using an application server property. Refer to the source file src/org/hsqldb/server/ to see the details.

Both HTTP Server and Servlet modes can only be accessed using the JDBC driver at the client end. They do not provide a web front end to the database. The Servlet mode can serve only a single database.

Please note that you do not normally use this mode if you are using the database engine in an application server. In this situation, connections to a catalog are usually made in-process, or using an external HSQL Server instance.

Server and Web Server Properties

Properties files for running the servers are not created automatically. You should create your own files that contain pairs for each property. The or files must be located in the directory where the command to run the org.hsqldb.server.Server class is issued.

In all properties files, values are case-sensitive. All values apart from names of files or pages are required in lowercase (e.g. server.silent=FALSE will have no effect, but server.silent=false will work). Supported properties and their default values (if any) are as follows:

Table 15.1. common server and webserver properties

server.database.0file:testthe catalog type, path and file name of the first database file to use
server.dbname.0""lowercase server alias for the first database file
server.database.nNO DEFAULTthe catalog type, path and file name of the n'th database file in use
server.dbname.nNO DEFAULTlowercase server alias for the n'th database file
server.silenttrueno extensive messages displayed on console
server.tracefalseJDBC trace messages displayed on console
server.addressNO DEFAULTIP address of server
server.tlsfalseWhether to encrypt network stream. If this is set to true, then in normal situations you will also need to set properties and, as documented elsewhere. The value of server.tls impacts the default value of server.port.
server.daemonfalseWhether the server is run as a daemon
server.remote_openfalseAllows opening a database path remotely when the first connection is made

In HyperSQL version 2.0, each server can serve an unlimited number of databases simultaneously. The server.database.0 property defines the filename / path whereas the server.dbname.0 defines the lowercase alias used by clients to connect to that database. The digit 0 is incremented for the second database and so on. Values for the server.database.n property can use the mem:, file: or res: prefixes and connection properties as discussed under CONNECTIONS. For example,


Properties or default values specific to are:

Table 15.2. server properties

server.port9001 (normal) or 554 (if TLS encrypted)TCP/IP port used for talking to clients. All databases are served on the same port.
server.no_system_exittrueno System.exit() call when the database is closed

Properties or default values specific to are:

Table 15.3. webserver properties

server.port80 (normal) or 443 (if TLS encrypted)TCP/IP port used for talking to clients
server.default_pageindex.htmlthe default web page for server
server.root./the location of served pages
.<extension>NO DEFAULTmultiple entries such as .html=text/html define the mime types of the static files served by the web server. See the source for src/org/hsqldb/server/ for a list.

An example of the contents of a file is given below:




In the above example, the file indicates that the server provides access to 3 different databases. Two of the databases are file based, while the third is all in memory. The aliases for the databases that the users connect to are accounts, enrolments and quickdb.

All the above properties and their values can be specified on the command line to start the server by omitting the server. prefix. If a property/value pair is specified on the command line, it overrides the property value specified in the or file.


Upgrading: If you have existing custom properties files, change the values to the new naming convention. Note the use of digits at the end of server.database.n and server.dbname.n properties.

Starting a Server from your Application

If you want to start the server from within your application, as opposed to the command line or batch files, you should create an instance of Server or Web Server, then assign the properties and start the Server. An working example of this can be found in the org.hsqldb.test.TestBase source. The example below sets the same properties as in the file example.

 HsqlProperties p = new HsqlProperties();
 // set up the rest of properties

 // alternative to the above is
 Server server = new Server();
 server.setLogWriter(null); // can use custom writer
 server.setErrWriter(null); // can use custom writer

Shutting down a Server from your Application

To shut down the server, you can execute the SQL "SHUTDOWN" statement on the server databases. When you start the server from your application and keep a reference to the Java Server object, you can also shut it down programatically. Calling the shutdownCatalogs(int shutdownMode) method of org.hsqldb.server.Server closes all the open databases, which results in server shutdown. The parameter value is normally 1, which indicates normal shutdown. Other modes of shutdown, such as SHUTDOWN IMMEDIATELY are also supported. See the javadoc for org.hsqldb.server.Server. See the example below:


The Server object has several alternative methods for setting databases and their public names. The server should be shutdown using the shutdown() method.

Allowing a Connection to Open or Create a Database

If the server.remote_open property is true, the Server works differently from the normal mode. In this mode, it is not necessary to have any databases listed as server.database.0 etc. in the Server startup properties. If there are databases listed, they are opened as normal. The server does not shutdown when the last database is closed.

In this mode, a connection can be established to a database that is not open or does not exist. The server will open the database or create it, then return a connection to the database.

The connection URL must include the path to the database, separated with a semicolon from the alias. In the example below, the database path specified as file:C:/files/mydatabase is opened and the database alias xdb is assigned to the database. After this, the next connection to the specified alias will connect to the same database. Any database path on the URL is ignored if the alias is serving a database.

The database path can point to a file: or mem: database.

If you use database properties on the URL, these properties are used when the new database is created. If no database properties are used on the URL, you can also specify the path with filepath=<path>. Examples below:

Connection c = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/xdb;file:C:/files/mydatabase", "SA", "");
Connection c = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/xdb;mem:test;sql.enforce_types=true", "SA", "");
Connection c = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/xdb;filepath=file:C:/files/mydatabase", "SA", "");

Specifying Database Properties at Server Start

Each database started by a Server has its own URL. When new databases are created by the server, the database properties for each of the new database can be appended to the database URL. Examples below:

// example in server.propertie file

// example for setting the property programatically
 HsqlProperties p = new HsqlProperties();

The specified properties apply only to a new database. They have no effect on an existing database apart from a few properties such as readonly listed in the Properties chapter.

TLS Encryption

Listener TLS Support (a. k. a. SSL)

Blaine Simpson

The HSQL Development Group

$Revision: 6692 $


This section explains how to encrypt the stream between JDBC network clients and HyperSQL Listeners. If you are running an in-process (non-Listener) setup, this chapter does not apply to you.


Hsqldb TLS Support Requirements

  • Java 4 and greater versions support JSSE.

  • A JKS keystore containing a private key, in order to run a Listener.

  • If you are running the listener side, then you'll need to run a HSQLDB Server or WebServer Listener instance. It doesn't matter if the underlying database catalogs are new, and it doesn't matter if you are making a new Listener configuration or encrypting an existing Listener configuration. (You can turn encryption on and off at will).

  • You need a HSQLDB jar file that was built with JSSE present. If you obtained your HSQLDB distribution from us, you are all set, because we build with Java 1.4 or later (which contains JSSE).

Encrypting your JDBC connection

At this time, only 1-way, server-cert encryption is tested.


Just use one of the following protocol prefixes.

Hsqldb TLS URL Prefixes

  • jdbc:hsqldb:hsqls://

  • jdbc:hsqldb:https://

The latter will only work for clients running with Java 1.4 or later.

If the listener you wish to connect to is using a certificate approved by your default trust keystore, then there is nothing else to do. If not, then you need to tell Java to "trust" the server cert. (It's a slight over-simplification to say that if the server certificate was purchased, then you are all set; if somebody "signed their own" certificate by self-signing or using a private ca certificate, then you need to set up trust).

First, you need to obtain the cert (only the "public" part of it). Since this cert is passed to all clients, you could obtain it by writing a Java client that dumps it to file, or perhaps by using openssl s_client. Since in most cases, if you want to trust a non-commercial cert, you probably have access to the server keystore, I'll show an example of how to get what you need from the server-side JKS keystore.

You may already have an X509 cert for your server. If you have a server keystore, then you can generate a X509 cert like this.

Example 15.1. Exporting certificate from the server's keystore

 keytool -export -keystore -alias existing_alias -file server.cer

In this example, server.cer is the X509 certificate that you need for the next step.

Now, you need to add this cert to one of the system trust keystores or to a keystore of your own. See the Customizing Stores section in JSSERefGuide.html to see where your system trust keystores are. You can put private keystores anywhere you want to. The following command will add the cert to an existing keystore, or create a new keystore if doesn't exist.

Example 15.2. Adding a certificate to the client keystore

 keytool -import -trustcacerts -keystore -alias new_alias -file server.cer

If you are making a new keystore, you probably want to start with a copy of your system default keystore which you can find somewhere under your JAVA_HOME directory (typically jre/lib/security/cacerts for a JDK, but I forget exactly where it is for a JRE).

Unless your OS can't stop other people from writing to your files, you probably do not want to set a password on the trust keystore.

If you added the cert to a system trust store, then you are finished. Otherwise you will need to specify your custom trust keystore to your client program. The generic way to set the trust keystore is to set the system property every time that you run your client program. For example

Example 15.3. Specifying your own trust store to a JDBC client

 java -jar /path/to/hsqldb.jar dest-urlid

This example runs the program SqlTool. SqlTool has built-in TLS support however, so, for SqlTool you can set truststore on a per-urlid basis in the SqlTool configuration file.

Note: The hostname in your database URL must match the Common Name of the server's certificate exactly. That means that if a site certificate is, you cannot use jdbc:hsqldb:hsqls://localhost or jdbc:hsqldb:hsqls:// to connect to it.

If you want more details on anything, see JSSERefGuide.html on Sun's site, or in the subdirectory docs/guide/security/jsse of your Java SE docs.

Server-Side (Listener-Side)

Get yourself a JKS keystore containing a private key. Then set properties server.tls, and in your or file. Set server.tls to true, to the path of the private key JKS keystore, and to the password (of both the keystore and the private key record-- they must be the same). If you specify relative file path values, they will be resolved relative to the ${user.dir} when the JRE is started.


If you set any password in a .properties (or any other) file, you need to restrict access to the file. On a good operating system, you can do this like so:

 chmod 600 path/to/

The values and behavior of the system.* settings above match the usage documented for and in the JSSE docs.


Before version 2.0, HyperSQL depended on directly setting the corresponding JSSE properties. The new idiom is more secure and easier to manage. If you have an old password in a UNIX init script config file, you should remove it.

Making a Private-key Keystore

There are two main ways to do this. Either you can use a certificate signed by a certificate authority, or you can make your own. One thing that you need to know in both cases is, the Common Name of the cert has to be the exact hostname that JDBC clients will use in their database URL.

CA-Signed Cert

I'm not going to tell you how to get a CA-signed SSL certificate. That is well documented at many other places.

Assuming that you have a standard pem-style private key certificate, here's how you can use openssl and the program DERImport to get it into a JKS keystore.

Because I have spent a lot of time on this document already, I am just giving you an example.

Example 15.4. Getting a pem-style private key into a JKS keystore

 openssl pkcs8 -topk8 -outform DER -in Xpvk.pem -inform PEM -out Xpvk.pk8 -nocrypt

 openssl x509 -in Xcert.pem -out Xcert.der -outform DER

 java DERImport new.keystore NEWALIAS Xpvk.pk8 Xcert.der


Make sure to set the password of the key exactly the same as the password for the keystore!

You need the program DERImport.class of course. Do some internet searches to find or DERImport.class and download it.

If DERImport has become difficult to obtain, I can write a program to do the same thing-- just let me know.

Non-CA-Signed Cert

Run man keytool or see the Creating a Keystore section of JSSERefGuide.html.

Automatic Server or WebServer startup on UNIX

If you are on UNIX and want to automatically start and stop a Server or WebServer running with encryption, set the and properties as instructed above, and follow the instructions in the HyperSQL on UNIX chapter, paying close attention to the TLS-related comments in the template config file.

If you are using a private server certificate, make sure to also set the trust store filepath for relevant urlids in your RC file, as explained in the sample config file.

Network Access Control

(Server ACLs)

JDBC connections will always be denied if the supplied user and password are not found in the target catalog. But an HyperSQL listener can also restrict access at the listener level, even protecting private catalogs which have insecure (or default) passwords. If you have an in-process setup, this section of the Guide doesn't apply to you.

Many (in fact, most) distributed database applications don't have application clients connect directly to the database, but instead encapsulate access in a controlling process. For example, a web app will usually access the data source on behalf of users, with end-user web browsers never accessing the database directly. In these cases and others, the security benefits of restricting listener access to specific source addresses is well worth the effort. ACLs work by restricting access according to the source address of the incoming connection request. This is efficient because the database engine never even gets the request until it is approved by the ACL filter code.

The sample file sample/acl.txt in your HyperSQL distribution explains how to write an ACL file.

# $Id: acl.txt 536 2008-12-05 14:55:10Z unsaved $

# Sample HyperSQL Network Listener ACL file.
# Specify "allow" and "deny" rules
# For address specifications, individual addresses, host names, and
# network addresses with /bit suffix are allowed, but read the caveat about
# host names below, under the sample "localhost" rule.

# Blank lines ignored.
   # Lines with # as the first non-whitespace character are ignored.

allow 2001:db8::/32
# Allow this 32-bit ipv4 subnet

allow localhost
# You should use numerical addresses in ACL files, unless you are certain that
# the name will always be known to your network address resolution system
# (assume that you will lose Internet connectivity at some time).
# With a default name resolution setup on UNIX, you are safe to use names
# defined in your /etc/hosts file.

# Deny a single IP address.
# In our example, is our local, organizational network.
# is the IP address of our Intern's PC.
# The Intern does not have permission to access our databases directly.


# Any ipv4 or ipv6 candidate address not matched above will be denied

You put your file wherever it is convenient for you, and specify that path with the property server.acl or webserver.acl in your or file (depending on whether your listener instance is a Server or WebServer). You can specify the ACL file path with an absolute or relative path. If you use a relative path, it must be relative to the .properties file. It's often convenient to name the ACL file acl.txt, in the same directory as your .properties file and specify the property value as just acl.txt. This file name is intuitive, and things will continue to work as expected if you move or copy the entire directory.


If your Server or WebServer was started with a *.acl property, changes afterwards to the ACL file will be picked up immediately by your listener instance. You are advised to use the procedure below to prevent partial edits or mistakes from crippling your running server.

When you edit your ACL file, it is both more convenient and more secure to test it as explained here before activating it. You could, of course, test an ACL file by editing it in-place, then trying to connect to your listener with JDBC clients from various source addresses. Besides being mightily laborious and boring, with this method it is very easy to accidentally open access to all source addresses or to deny access to all users until you fix incorrect ACL entries.

The suggested method of creating or changing ACLs is to work with an inactive file (for new ACL files, just don't enable the *.acl property yet; for changing an existing file, just copy it to a temporary file and edit the temporary file). Then use the ServerAcl class to test it.

Example 15.5. Validating and Testing an ACL file

 java -cp path/to/hsqldb.jar org.hsqldb.server.ServerAcl path/to/acl.txt

If the specified ACL file fails validation, you will be given details about the problem. Otherwise, the validated rules will be displayed (including the implicit, default deny rules). You then type in host names and addresses, one-per-line. Each name or address is tested as if it were a HyperSQL network client address, using the same exact method that the HyperSQL listener will use. (HyperSQL listeners use this same ServerAcl class to test incoming source addresses). ServerAcl will report the rule which matches and whether access is denied or allowed to that address.

If you have edited a copy of an existing ACL file (as suggested above), then overwrite your live ACL file with your new, validated ACL file. I.e., copy your temp file over top of your live ACL file.

ServerAcl can be run in the same exact way described above, to troubleshoot runtime access issues. If you use an ACL file and a user or application can't get a connection to the database, you can run ServerAcl to quickly and definitively find if the client is being prohibited by an ACL rule.

$Revision: 6752 $