You can customize the code that the Slice-to-Java compiler produces by annotating your Slice definitions with metadata. This section describes how metadata influences several aspects of the generated Java code.
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By default, the scope of a Slice definition determines the package of its mapped Java construct. A Slice type defined in a module hierarchy is mapped to a type residing in the equivalent Java package.
There are times when applications require greater control over the packaging of generated Java classes. For instance, a company may have software development guidelines that require all Java classes to reside in a designated package. One way to satisfy this requirement is to modify the Slice module hierarchy so that the generated code uses the required package by default. In the example below, we have enclosed the original definition of
Workflow::Document in the modules
com::acme so that the compiler will create the class in the
There are two problems with this workaround:
- It incorporates the requirements of an implementation language into the application's interface specification.
- Developers using other languages, such as C++, are also affected.
The Slice-to-Java compiler provides a better way to control the packages of generated code through the use of metadata. The example above can be converted as follows:
The metadata directive
java:package:com.acme instructs the compiler to generate all of the classes resulting from definitions in module
Workflow into the Java package
com.acme. The net effect is the same: the class for
Document is generated in the package
com.acme.Workflow. However, we have addressed the two shortcomings of the first solution by reducing our impact on the interface specification: the Slice-to-Java compiler recognizes the package metadata directive and modifies its actions accordingly, whereas the compilers for other language mappings simply ignore it.
java:package directive can also be applied as file metadata, in which case it serves as the default directive unless overridden by module metadata.
Java Package Configuration Properties
Using metadata to alter the default package of generated classes has ramifications for the Ice run time when unmarshaling exceptions and concrete class types. The Ice run time dynamically loads generated classes by translating their Slice type IDs into Java class names. For example, the Ice run time translates the Slice type ID
::Workflow::Document into the class name
However, when the generated classes are placed in a user-specified package, the Ice run time can no longer rely on the direct translation of a Slice type ID into a Java class name, and therefore requires additional configuration so that it can successfully locate the generated classes. Two configuration properties are supported:
Associates a top-level Slice module with the package in which it was generated.
Only top-level module names are allowed; the semantics of file metadata prevent a nested module from being generated into a different package than its enclosing module.
Specifies a default package to use if other attempts to load a class have failed.
The behavior of the Ice run time when unmarshaling an exception or concrete class is described below:
- Translate the Slice type ID into a Java class name and attempt to load the class.
- If that fails, extract the top-level module from the type ID and check for an
Ice.Packageproperty with a matching module name. If found, prepend the specified package to the class name and try to load the class again.
- If that fails, check for the presence of
Ice.Default.Package. If found, prepend the specified package to the class name and try to load the class again.
- If the class still cannot be loaded, the instance may be sliced.
Continuing our example from the previous section, we can define the following property:
Alternatively, we could achieve the same result with this property:
Custom Types in Java
One of the more powerful applications of metadata is the ability to tailor the Java mapping for sequence and dictionary types to match the needs of your application.
Custom Type Metadata in Java
The metadata for specifying a custom type has the following format:
The formal type is optional; the compiler uses a default value if one is not defined. The instance type must satisfy an is-A relationship with the formal type: either the same class is specified for both types, or the instance type must be derived from the formal type.
The Slice-to-Java compiler generates code that uses the formal type for all occurrences of the modified Slice definition except when the generated code must instantiate the type, in which case the compiler uses the instance type instead.
The compiler performs no validation on your custom types. Misspellings and other errors will not be apparent until you compile the generated code.
Defining a Custom Sequence Type in Java
Although the default mapping of a sequence type to a native Java array is efficient and typesafe, it is not always the most convenient representation of your data. To use a different representation, specify the type information in a metadata directive, as shown in the following example:
It is your responsibility to use a type parameter for the Java class (
String in the example above) that is the correct mapping for the sequence's element type.
The compiler requires the formal type to implement
E is the Java mapping of the element type. If you do not specify a formal type, the compiler uses
java.util.List<E> by default.
Note that extra care must be taken when defining custom types that contain nested generic types, such as a custom sequence whose element type is also a custom sequence. The Java compiler strictly enforces type safety, therefore any compatibility issues in the custom type metadata will be apparent when the generated code is compiled.
Defining a Custom Dictionary Type in Java
The default instance type for a dictionary is
java.util.HashMap<K, V>, where
K is the Java mapping of the key type and
V is the Java mapping of the value type. If the semantics of a
HashMap are not suitable for your application, you can specify an alternate type using metadata as shown in the example below:
It is your responsibility to use type parameters for the Java class (
String in the example above) that are the correct mappings for the dictionary's key and value types.
The compiler requires the formal type to implement
java.util.Map<K, V>. If you do not specify a formal type, the compiler uses this type by default.
Note that extra care must be taken when defining dictionary types that contain nested generic types, such as a dictionary whose element type is a custom sequence. The Java compiler strictly enforces type safety, therefore any compatibility issues in the custom type metadata will be apparent when the generated code is compiled.
Using Custom Type Metadata in Java
You can define custom type metadata in a variety of situations. The simplest scenario is specifying the metadata at the point of definition:
Defined in this manner, the Slice-to-Java compiler uses
java.util.List<String> (the default formal type) for all occurrences of
java.util.LinkedList<String> when it needs to instantiate
You may also specify a custom type more selectively by defining metadata for a data member, parameter or return value. For instance, the mapping for the original Slice definition might be sufficient in most situations, but a different mapping is more convenient in particular cases. The example below demonstrates how to override the sequence mapping for the data member of a structure as well as for several operations:
As you might expect, modifying the mapping for an operation's parameters or return value may require the application to manually convert values from the original mapping to the modified mapping. For example, suppose we want to invoke the
modifiedInParam operation. The signature of its proxy operation is shown below:
The metadata changes the mapping of the
seq parameter to
java.util.List, which is the default formal type. If a caller has a
StringSeq value in the original mapping, it must convert the array as shown in the following example:
Although we specified the instance type
java.util.ArrayList<String> for the parameter, we are still able to pass the result of
asList because its return type (
java.util.List<String>) is compatible with the parameter's formal type declared by the proxy method. In the case of an operation parameter, the instance type is only relevant to a servant implementation, which may need to make assumptions about the actual type of the parameter.
Buffer Types in Java
You can annotate sequences of certain primitive types with the
java:buffer metadata tag to change the mapping to use subclasses of
java.nio.Buffer. This mapping provides several benefits:
- You can pass a buffer to a Slice API instead of creating and filling a temporary array
- If you need to pass a portion of an existing array, you can wrap it with a buffer and avoid an extra copy
- Receiving buffers during a Slice operation also avoids copying by directly referencing the data in Ice's unmarshaling buffer
To use buffers safely, applications must disable caching by setting
Ice.CacheMessageBuffers to zero.
The following table lists each supported Slice primitive type with its corresponding mapped class:
java:buffer tag can be applied to the initial definition of a sequence, in which case the mapping uses the buffer type for all occurrences of that sequence type:
We can construct an
Observation as follows:
java:buffer tag can also be applied in more limited situations to override a sequence's normal mapping:
In this example,
ByteSeq maps by default to a
byte array, but we've overridden the mapping to use a buffer when this type is used as a data member in
Page and as the return value of the
decode operation; the input parameter to
decode uses the default array mapping.
The Java mapping optionally generates JavaBean-style methods for the data members of class, structure, and exception types.
JavaBean Generated Methods
For each data member
val of type
T, the mapping generates the following methods:
The mapping generates an additional method if
T is the
T is a sequence type with an element type
E, two methods are generated to provide direct access to elements:
Note that these element methods are only generated for sequence types that use the default mapping.
The Slice-to-Java compiler considers it a fatal error for a JavaBean method of a class data member to conflict with a declared operation of the class. In this situation, you must rename the operation or the data member, or disable the generation of JavaBean methods for the data member in question.
The JavaBean methods are generated for a data member when the member or its enclosing type is annotated with the
java:getset metadata. The following example demonstrates both styles of usage:
JavaBean methods are generated for all members of struct
S and exception
E, but for only one member of class
C. Relevant portions of the generated code are shown below:
The Slice-to-Java compiler computes a default value for the
serialVersionUID member of Slice classes, exceptions and structures. If you prefer, you can override this value using the
java:serialVersionUID metadata, as shown below:
The specified value will be used in place of the default value in the generated code:
By using this metadata, the application assumes responsibility for updating the UID whenever changes to the Slice definition affect the serializable state of the type.