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Basic Java Mapping for Operations

As we saw in the mapping for interfaces, for each operation on an interface, the proxy class contains a corresponding member function with the same name. To invoke an operation, you call it via the proxy. For example, here is part of the definitions for our file system:

module Filesystem
{
    interface Node
    {
        idempotent string name();
    }
    // ...
}

The name operation returns a value of type string. Given a proxy to an object of type Node, the client can invoke the operation as follows:

NodePrx node = ...;             // Initialize proxy
String name = node.name();      // Get name via RPC

This illustrates the typical pattern for receiving return values: return values are returned by reference for complex types, and by value for simple types (such as int or double).

Normal and `idempotent` Operations in Java

You can add an idempotent qualifier to a Slice operation. As far as the signature for the corresponding proxy method is concerned, idempotent has no effect. For example, consider the following interface:

interface Example
{
                string op1();
    idempotent  string op2();
}

The proxy interface for this is:

public interface ExamplePrx extends ObjectPrx
{
    String op1();
    String op2();
}

Because idempotent affects an aspect of call dispatch, not interface, it makes sense for the two methods to be mapped the same.

Passing Parameters in Java

In Parameters in Java

The parameter passing rules for the Java mapping are very simple: parameters are passed either by value (for simple types) or by reference (for complex types and type string). Semantically, the two ways of passing parameters are identical: it is guaranteed that the value of a parameter will not be changed by the invocation.

Here is an interface with operations that pass parameters of various types from client to server:

struct NumberAndString
{
    int x;
    string str;
}

sequence<string> StringSeq;

dictionary<long, StringSeq> StringTable;

interface ClientToServer
{
    void op1(int i, float f, bool b, string s);
    void op2(NumberAndString ns, StringSeq ss, StringTable st);
    void op3(ClientToServer* proxy);
}

The Slice compiler generates the following proxy for these definitions:

public interface ClientToServerPrx extends ObjectPrx
{
    void op1(int i, float f, boolean b, String s);
    void op2(NumberAndString ns, String[] ss, java.util.Map<Long, String[]> st);
    void op3(ClientToServerPrx proxy);
}

Given a proxy to a ClientToServer interface, the client code can pass parameters as in the following example:

ClientToServerPrx p = ...;              // Get proxy...

p.op1(42, 3.14f, true, "Hello world!"); // Pass simple literals

int i = 42;
float f = 3.14f;
boolean b = true;
String s = "Hello world!";
p.op1(i, f, b, s);                      // Pass simple variables

NumberAndString ns = new NumberAndString();
ns.x = 42;
ns.str = "The Answer";
String[] ss = { "Hello world!" };
java.util.Map<Long, String[]> st = new java.util.HashMap<Long, String[]>();
st.put(0, ns);
p.op2(ns, ss, st);                      // Pass complex variables

p.op3(p);                               // Pass proxy

Out Parameters in Java

The mapping for an operation depends on how many values it returns, including out parameters and a non-void return value:

Zero values
The corresponding Java method returns void. For the purposes of this discussion, we're not interested in these operations.

One value
The corresponding Java method returns the mapped type, regardless of whether the Slice definition of the operation declared it as a return value or as an out parameter. Consider this example:
interface I { string op1(); void op2(out string name); }
The mapping generates corresponding methods with identical signatures:
interface IPrx extends ObjectPrx { String op1(); String op2(); }
Multiple values
The Slice-to-Java translator generates an extra nested class to hold the results of an operation that returns multiple values. The class is nested in the mapped interface (not the proxy interface) and has the name OpResult, where Op represents the name of the operation. The leading character of the class name for a "result class" is always capitalized. The values of out parameters are provided in corresponding data members of the same names. If the operation declares a return value, its value is provided in the data member named returnValue. If an out parameter is also named returnValue, the data member to hold the operation's return value is named _returnValue instead. The result class defines an empty constructor as well as a "one-shot" constructor that accepts and assigns a value for each of its data members. The corresponding Java method returns the result class type.

Here are the same Slice definitions we saw earlier, but this time with all parameters being passed in the out direction, along with one additional operation to better demonstrate the mapping:

struct NumberAndString
{
    int x;
    string str;
}

sequence<string> StringSeq;

dictionary<long, StringSeq> StringTable;

interface ServerToClient
{
    void op1(out int i, out float f, out bool b, out string s);
    void op2(out NumberAndString ns,
             out StringSeq ss,
             out StringTable st);
    void op3(out ServerToClient* proxy);
    StringSeq op4(out string returnValue);
}

The Slice compiler generates the following code for these definitions:

public interface ServerToClientPrx extends ObjectPrx
{
    ServerToClient.Op1Result op1();
    ServerToClient.Op2Result op2();
    ServerToClientPrx op3();
    ServerToClient.Op4Result op4();
}

public interface ServerToClient extends ...
{
    public static class Op1Result
    {
        public Op1Result() {}
        public Op1Result(int i, float f, boolean b, String s)
        {
            this.i = i;
            this.f = f;
            this.b = b;
            this.s = s;
        }
        public int i;
        public float f;
        public boolean b;
        public String s;
    }
 
    public static class Op2Result
    {
        public Op2Result() {}
        public Op2Result(NumberAndString ns, String[] ss, java.util.Map<java.lang.Long, String[]> st)
        {
            this.ns = ns;
            this.ss = ss;
            this.st = st;
        }
        public NumberAndString ns;
        public String[] ss;
        public java.util.Map<java.lang.Long, String[]> st;
    }
 
    public static class Op4Result
    {
        public Op4Result() {}
        public Op4Result(String[] _returnValue, String returnValue)
        {
            this._returnValue = _returnValue;
            this.returnValue = returnValue;
        }
        public String[] _returnValue;
        public String returnValue;
    }
}

We need to point out several things here:

Result classes are generated for op1, op2 and op4 because they return multiple values
The result classes are generated as nested classes of interface ServerToClient, and not ServerToClientPrx
op4 declares an out parameter named returnValue, therefore Op4Result declares a data member named _returnValue to hold the operation's return value
No result class is necessary for op3 because it only returns one value; the mapped Java method declares a return type of ServerToClientPrx even though the Slice operation declared it as an out parameter

Null Parameters in Java

Some Slice types naturally have "empty" or "not there" semantics. Specifically, sequences, dictionaries, and strings all can be null, but the corresponding Slice types do not have the concept of a null value. To make life with these types easier, whenever you pass null as a parameter or return value of type sequence, dictionary, or string, the Ice run time automatically sends an empty sequence, dictionary, or string to the receiver.

This behavior is useful as a convenience feature: especially for deeply-nested data types, members that are sequences, dictionaries, or strings automatically arrive as an empty value at the receiving end. This saves you having to explicitly initialize, for example, every string element in a large sequence before sending the sequence in order to avoid NullPointerException. Note that using null parameters in this way does not create null semantics for Slice sequences, dictionaries, or strings. As far as the object model is concerned, these do not exist (only empty sequences, dictionaries, and strings do). For example, whether you send a string as null or as an empty string makes no difference to the receiver: either way, the receiver sees an empty string.

Optional Parameters in Java

The mapping uses standard Java types to encapsulate optional parameters:

java.util.OptionalDouble
The mapped type for an optional double.

java.util.OptionalInt
The mapped type for an optional int.

java.util.OptionalLong
The mapped type for an optional long.

java.util.Optional<T>
The mapped type for all other Slice types.

Optional return values and output parameters are mapped to instances of the above classes, depending on their types. For operations with optional in parameters, the proxy provides a set of overloaded methods that accept them as optional values, and another set of methods that accept them as required values. Consider the following operation:

optional(1) int execute(optional(2) string params);

The mapping for this operation is shown below:

java.util.OptionalInt execute(String params);
java.util.OptionalInt execute(java.util.Optional<String> params);

For cases where you are passing values for all of the optional in parameters, it is more efficient to use the required mapping and avoid creating temporary optional values.

A client can invoke execute as shown below:

java.util.OptionalInt i;

i = proxy.execute("--file log.txt");                        // required mapping
i = proxy.execute(java.util.Optional.of("--file log.txt")); // optional mapping
i = proxy.execute(java.util.Optional.empty());              // params is unset
 
if(i.isPresent())
{
    System.out.println("value = " + i.get());
}

Passing null where an optional value is expected is equivalent to passing an instance whose value is unset.

Java's optional classes do not consider null to be a legal value. Consider this example:

class Data
{
    ...
}
 
interface Repository
{
    void addOptional(optional(1) Data d);
    void addRequired(Data d);
}

The Ice encoding allows null values for class instances, so you can pass null to addRequired and the server will receive it as null. However, there's no way to pass an "optional value set to null" in the Java mapping. Passing null to addOptional is equivalent to passing the value of java.util.Optional.ofNullable((T)null), which is equivalent to passing the value of java.util.Optional.empty(). In either case, the server will receive it as an optional whose value is not present.

A well-behaved program must not assume that an optional parameter always has a value. Calling get on an optional instance for which no value is set raises java.util.NoSuchElementException.

Exception Handling in Java

Any operation invocation may throw a run-time exception and, if the operation has an exception specification, may also throw user exceptions. Suppose we have the following simple interface:

exception Tantrum
{
    string reason;
}

interface Child
{
    void askToCleanUp() throws Tantrum;
}

Slice exceptions are thrown as Java exceptions, so you can simply enclose one or more operation invocations in a try-catch block:

ChildPrx child = ...;   // Get child proxy...

try
{
    child.askToCleanUp();
}
catch(Tantrum t)
{
    System.out.write("The child says: ");
    System.out.writeln(t.reason);
}

Typically, you will catch only a few exceptions of specific interest around an operation invocation; other exceptions, such as unexpected run-time errors, will typically be handled by exception handlers higher in the hierarchy. For example:

public class Client
{
    static void run()
    {
        ChildPrx child = ...;   // Get child proxy...
        try
        {
            child.askToCleanUp();
        }
        catch(Tantrum t)
        {
            System.out.print("The child says: ");
            System.out.println(t.reason);
            child.scold();          // Recover from error...
        }
        child.praise();             // Give positive feedback...
    }

    public static void main(String[] args)
    {
        try
        {
            // ...
            run();
            // ...
        }
        catch(LocalException e)
        {
            e.printStackTrace();
        }
        catch(UserException e)
        {
            System.err.println(e.getMessage());
        }
    }
}

This code handles a specific exception of local interest at the point of call and deals with other exceptions generically. (This is also the strategy we used for our first simple application.)

Exceptions and Out-Parameters

For the Java Compat mapping, the Ice run time makes no guarantees about the state of out parameters when an operation throws an exception: the parameter may still have its original value or may have been changed by the operation's implementation in the target object. In other words, for out parameters, Ice provides the weak exception guarantee [1] but does not provide the strong exception guarantee.

This is done for reasons of efficiency: providing the strong exception guarantee would require more overhead than can be justified.

References

Sutter, H. 1999. Exceptional C++: 47 Engineering Puzzles, Programming Problems, and Solutions. Reading, MA: Addison-Wesley.