# Calling Java from MATLAB

So far no one has taken me up on the extra credit from the Interactive Web Pages post. I thought the problem of displaying a figure without an image file was too interesting to pass up, so I figured out the solution myself and put it up on the file exchange. The second challenge involving ActiveX will remain open until I get chance to write up its solution. Or, if you don't have the time to do that, you can still win a t-shirt by sending us your desktop.

My solution to the image challenge involves encoding binary information from an image file as text through the base64 scheme. At first I thought to implement the base64 algorithm myself in MATLAB, but I learned my lesson the hard way a few years ago writing my own md5 hash-computing program (don't ask). This time I wanted to use a readily-available solution. Since there is no toolbox function for this, I decided to take advantage of a Java library since (a) I could use the same library on every platform (instead of needing a different pre-compiled binary) and (b) I am pretty comfortable with Java. My solution takes advantage of the freely available Apache Commons Codec package.

Using Java from a MATLAB program is as simple as using a MATLAB function, particularly if you are used to using MATLAB package functions. You can use any public method in the Java SE API from the command line. The running version of the JRE depends on your version of MATLAB and operating system and determines what API functionality is available from MATLAB. To determine the Java version use the command version('-java'). To change the Java version (not recommended) you can follow these instructions.

For instance instead of MATLAB's str2num you could use: Double's parseDouble.

str2num('2.3')
java.lang.Double.parseDouble('2.3')
ans =

2.3000

ans =

2.3000



Of course, this is a trivial example. Many of our functions such as xmlread make use of more complicated series of calls to java objects. The payoff in the sendmail function comes at the end using the Transport object to send a pre-constructed MimeMessage.

javax.mail.Transport.send(msg)


My base64 program does not use a class from the standard JRE (in fact, neither does sendmail) but instead uses a third party library (Apache Commons Codec) that I happen to know ships with MATLAB and is already available on the classpath.

encoder = org.apache.commons.codec.binary.Base64;
base64string = char(encoder.encode(bytes))';


These examples has three things about using Java that I want to address so you can use other Java libraries in MATLAB: (1) the class path, (2) calling syntax, and (3) data types.

Java Class Path
MATLAB maintains a path for Java classes separate from the search path. That means even if you have a .class or .jar file on the MATLAB path, unless you use javaaddpath you will not be able to use it. To see what is currently on the path use javaclasspath. Running this command you will show you a long list of files that ship with matlab called the Static Class Path and then you'll see the Dynamic Class Path. The dynamic class path is where classes added to the path with javaaddpath will be placed. They can be removed with javarmpath and have to actively reloaded each session of matlab. The static class path comes from \$matlabroot\toolbox\local\classpath.txt and are only added to the class path at MATLAB startup. The ones listed here by default are all the Java classes we write (everything in the com.mathworks package). Calling any of these classes directly is not supported and likely to break on a release-to-release basis, but there are resources on the web that make use of these effectively.

Calling Java Functions
Once you add a class to the classpath and it shows up in the javaclasspath, you can use it from the Command Line or from a MATLAB-file. To do that you can use the fully-qualified name (e.g. javax.swing.JTable) or like with MATLAB packages, you can use the import statement to just call the class name (e.g. import('javax.swing.*'),JTable ).

Data Types
Looking at my string to double example you'll see that I passed a MATLAB char array to the java function and got back a MATLAB double, even though methodsview('java.lang.Double') shows that parseDouble takes a java.lang.String as its input. MATLAB will automatically convert primitive types when calling Java methods, as well as convert char arrays to java.lang.String objects when used as inputs. As you can see in my base64 example, I have to explicitly convert the java.lang.String returned by encoder.encode() to a MATLAB char array.

Those are the basics of calling Java. In future posts I hope to discuss memory management with Java (a topic of some recent technical support queries) as well as threading issues, as related to GUIs.

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