Dynamic Invocation and Dispatch in Java

This page describes the Java mapping for the ice_invoke proxy function and the Blobject class.

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ice_invoke in Java

The mapping for ice_invoke is shown below:

Java
boolean ice_invoke(
    String operation,
    Ice.OperationMode mode,
    byte[] inParams,
    Ice.ByteSeqHolder outParams
);

Another overloading of ice_invoke (not shown) adds a trailing argument of type Ice.Context.

As an example, the code below demonstrates how to invoke the operation op, which takes no in parameters:

Java
Ice.ObjectPrx proxy = ...
try {
    Ice.ByteSeqHolder outParams = new Ice.ByteSeqHolder();
    if (proxy.ice_invoke("op", Ice.OperationMode.Normal, null, outParams)) {
        // Handle success
    } else {
        // Handle user exception
    }
} catch (Ice.LocalException ex) {
    // Handle exception
}

As a convenience, the Ice run time accepts an empty or null byte sequence when there are no input parameters and internally translates it into an empty encapsulation. In all other cases, the value for inParams must be an encapsulation of the encoded parameters.

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Using Streams with ice_invoke in Java

The streaming interfaces provide the tools an application needs to dynamically invoke operations with arguments of any Slice type. Consider the following Slice definition:

Slice
module Calc {
    exception Overflow {
        int x;
        int y;
    };
    interface Compute {
        idempotent int add(int x, int y)
            throws Overflow;
    };
};

Now let's write a client that dynamically invokes the add operation:

Java
Ice.ObjectPrx proxy = ...
try {
    Ice.OutputStream out = Ice.Util.createOutputStream(communicator);
    out.startEncapsulation();
    out.writeInt(100); // x
    out.writeInt(-1);  // y
    out.endEncapsulation();
    byte[] inParams = out.finished();
 
    Ice.ByteSeqHolder outParams = new Ice.ByteSeqHolder();
    if (proxy.ice_invoke("add", Ice.OperationMode.Idempotent, inParams, outParams)) {
        // Handle success
        Ice.InputStream in = Ice.Util.createInputStream(communicator, outParams.value);
        in.startEncapsulation();
        int result = in.readInt();
        in.endEncapsulation();
        assert(result == 99);
    } else {
        // Handle user exception
    }
} catch (Ice.LocalException ex) {
    // Handle exception
}

You can see here that the input and output parameters are enclosed in encapsulations.

We neglected to handle the case of a user exception in this example, so let's implement that now. We assume that we have compiled our program with the Slice-generated code, therefore we can call throwException on the input stream and catch Overflow directly:

Java
    if (proxy.ice_invoke("add", Ice.OperationMode.Idempotent, inParams, outParams)) {
        // Handle success
        // ...
    } else {
        // Handle user exception
        Ice.InputStream in = Ice.Util.createInputStream(communicator, outParams.value);
        try {
            in.startEncapsulation();
            in.throwException();
        } catch (Calc.Overflow ex) {
            System.out.println("overflow while adding " + ex.x + " and " + ex.y);
        } catch (Ice.UserException ex) {
            // Handle unexpected user exception
        }
    }

This is obviously a contrived example: if the Slice-generated code is available, why bother using dynamic dispatch? In the absence of Slice-generated code, the caller would need to manually unmarshal the user exception, which is outside the scope of this manual.

As a defensive measure, the code traps Ice.UserException. This could be raised if the Slice definition of add is modified to include another user exception but this segment of code did not get updated accordingly.

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Subclassing Blobject in Java

Implementing the dynamic dispatch model requires writing a subclass of Ice.Blobject. We continue using the Compute interface to demonstrate a Blobject implementation:

Java
public class ComputeI extends Ice.Blobject {
    public boolean ice_invoke(
        byte[] inParams,
        Ice.ByteSeqHolder outParams,
        Ice.Current current)
    {
        // ...
    }
}

An instance of ComputeI is an Ice object because Blobject derives from Object, therefore an instance can be added to an object adapter like any other servant.

For the purposes of this discussion, the implementation of ice_invoke handles only the add operation and raises OperationNotExistException for all other operations. In a real implementation, the servant must also be prepared to receive invocations of the following Object operations:

  • string ice_id()
    Returns the Slice type ID of the servant's most-derived type.
  • StringSeq ice_ids()
    Returns a sequence of strings representing all of the Slice interfaces supported by the servant, including "::Ice::Object".
  • bool ice_isA(string id)
    Returns true if the servant supports the interface denoted by the given Slice type ID, or false otherwise. This operation is invoked by the proxy function checkedCast.

With that in mind, here is our simplified version of ice_invoke:

Java
    public boolean ice_invoke(
        byte[] inParams,
        Ice.ByteSeqHolder outParams,
        Ice.Current current)
    {
        if (current.operation.equals("add")) {
            Ice.Communicator communicator = current.adapter.getCommunicator();
            Ice.InputStream in = Ice.Util.createInputStream(communicator, inParams);
            in.startEncapsulation();
            int x = in.readInt();
            int y = in.readInt();
            in.endEncapsulation();
 
            Ice.OutputStream out = Ice.Util.createOutputStream(communicator);
            try {
                if (checkOverflow(x, y)) {
                    Calc.Overflow ex = new Calc.Overflow();
                    ex.x = x;
                    ex.y = y;
                    out.startEncapsulation();
                    out.writeException(ex);
                    out.endEncapsulation();
                    outParams.value = out.finished();
                    return false;
                } else {
                    out.startEncapsulation();
                    out.writeInt(x + y);
                    out.endEncapsulation();
                    outParams.value = out.finished();
                    return true;
                }
            } finally {
                out.destroy();
            }
        } else {
            Ice.OperationNotExistException ex = new Ice.OperationNotExistException();
            ex.id = current.id;
            ex.facet = current.facet;
            ex.operation = current.operation;
            throw ex;
        }
    }

If an overflow is detected, the code "raises" the Calc::Overflow user exception by calling writeException on the output stream and returning false, otherwise the return value is encoded and the function returns true.

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See Also