Tuesday, April 10, 2018

OSGi R7 Highlights: The Converter

One of the entirely new specifications in the OSGi R7 set of specs is the Converter specification. The converter can convert between many different Java types and can also be a highly convenient tool to give untyped data a proper API.

Let's dive in with some simple converter examples:

// Obtain the converter
Converter c = Converters.standardConverter();

// Simple scalar conversions
int i = c.convert("123").to(int.class);
UUID id = c.convert("067e6162-3b6f-4ae2-a171-2470b63dff00")
BigDecimal bd = c.convert(12345).to(BigDecimal.class);

Collections, arrays and maps can also be converted:

List<String> ls = Arrays.asList("978", "142", "-99");

// Convert a List of Strings to a Set of Integers, 
// note the use of TypeReference to capture the generics
Set<Integer> is = c.convert(ls)
                   .to(new TypeReference<Set<Integer>>(){});

// Convert between map-like structures
Dictionary<String, String> d = ...; // some dictionary
Map m = c.convert(d).to(Map.class);

// Even though properties contain String keys and values, you 
// can't assign them to a Map<String, String>, the converter 
// can make this easy...
Properties p = ...; // a properties object
Map<String, String> n = c.convert(p)
           .to(new TypeReference<Map<String,String>>(){});

The above examples show that many different types of conversions can be made with the converter, using the same convert(object).to(type) API.

Note that the converter is obtained by calling Converters.standardConverter() so by default it's not registered in the service registry. This makes the converter usable in both OSGi Framework as well as in plain Java environments.

Backing Objects

An interesting feature of the converter is that it can convert between maps and Java types such as interfaces, annotations and java beans. Together we call these map-like types. In certain cases the conversion can provide a live view over the backing object. 

For example, let's say you have an interface MyAPI:

interface MyAPI {
    long timeout();
    long timeout(long defval);

Now you have a map that contains a timeout key with some value, if you want to expose that via the MyAPI interface the converter can do this:

Map<String, Long> m = new HashMap<>();
m.put("timeout", 999L);
MyAPI o = c.convert(m).to(MyAPI.class);
System.out.println(o.timeout()); // prints 999
m.put("timeout", 1000L);         // update the underlying map
System.out.println(o.timeout()); // prints 1000

The other way around is also possible. For example, if you have a JavaBean that you need to view as a Map, then the converter can do this. In the following example, we see that in order for the converter to handle a source object as a JavaBean you need to specify the sourceAsBean() modifier. Additionally, live views for maps are not enabled by default and are enabled with the view() modifier.

MyBean bean = new MyBean();
Map<String, String> bm = c.convert(bean).sourceAsBean().view()
           .to(new TypeReference<Map<String,String>>(){});
System.out.println("Bean map:" + bm);
bean.setWidth(125)                    // update the bean
System.out.println("Bean map:" + bm); // map reflects the update

In addition to the map-like types mentioned above, the converter can handle DTOs and when an object exposes a method called getProperties(), it can take this as the source map as well.


Converting a map or properties object to a Java interface can be a really useful way to give some untyped configuration a well-defined API. Often you'd want to be able to specify some defaults for the configuration values that you're using, in case they are not specified. When converting to interfaces or annotations defaults can be specified.

When converting to an interface, the interface may define a one-argument method to specify the default, as is done with the MyAPI interface above. To specify the default pass it into the method:

Map<String, Long> m = new HashMap<>(); // empty map
MyAPI o = c.convert(m).to(MyAPI.class);

// default to 750 if no timeout is in the map

Handling defaults via an annotation is even a little neater, as the Java annotation syntax allows for the specification of defaults.     

@interface MyAnn {
    long timeout() default 100;

MyAnn a = c.convert(m2).to(MyAnn.class);
System.out.println(a.timeout()); // use default 100
m2.put("timeout", 500L);
System.out.println(a.timeout()); // now get the actual value 500

Customizing Converters

While the converter can convert between many types (see the spec chapter for the actual list of supported types) it doesn't understand every single Java type ever created. Let's say we have two custom types Foo and Bar. There is a conversion between the two, but the converter doesn't know about it. We can create a customized converter that, on top of its existing conversions, knows about our special conversion as well!

Let's say our custom classes look a bit like this. The Foo class can be represented as a string which you can obtain via its read() method. The Bar class can hold a byte array.

public class Foo {
    // ...
    public String read() {
        return val;

public static class Bar {
    byte[] buf;

    public static Bar create(byte[] b) {
        Bar bar = new Bar();
        bar.buf = b;
        return bar;

To create a converter that, on top of the existing conversions, knows about Foo and Bar, we use a converter builder and register a rule with that to do the conversion. In the example below, the Rule object declares the source and the target of the rule. The conversion is provided as a lambda:

Foo f = ...; // Some Foo object
Converter custom = c.newConverterBuilder()
    .rule(new Rule<Foo, Bar>(
        foo -> Bar.create(foo.read().getBytes())){})

// Now we can convert Foo to Bar!
Bar b = custom.convert(f).to(Bar.class);

Now the converter can convert our custom objects. An additional benefit is that the converter is an immutable object. You can customize an already customized converter by creating a new converter based on the previously created custom one.

Because the converter is immutable it's ideal for sharing, so once you've customized the perfect converter for your application, a good way to share it is by registering it in the OSGi Service Registry, and because the default converter isn't available in the service registry, your application components can simply look up the Converter service to get the one you registered for application wide usage.

Getting Started

This blog post doesn't cover all the details of the converter specification. See the spec chapter for more info on those. Like to get started straight away? At the time of this writing the specs are still under vote and not yet finally released. However, you can obtain a snapshot converter implementation from the OSGi Maven Repo: https://oss.sonatype.org/content/repositories/osgi/org/osgi/org.osgi.util.converter/ or you can build one yourself from the codebase in the Apache Felix project: https://svn.apache.org/repos/asf/felix/trunk/converter/converter.

Want to find out more about OSGi R7?

This is one post in a series of 12 focused on OSGi R7 Highlights.  Previous posts are:
  1. Proposed Final Draft Now Available
  2. Java 9 Support
  3. Declarative Services
  4. The JAX-RS Whiteboard
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