10.19 Using Slice Checksums

As described in Section 4.21, the Slice compilers can optionally generate check­sums of Slice definitions. For slice2java, the ‑‑checksum option causes the compiler to generate a new Java class that adds checksums to a static map member. Assuming we supplied the option ‑‑checksum Checksums to slice2java, the generated class Checksums.java looks like this:

public class Checksums {
    public static java.util.Map checksums;
}

The read-only map checksums is initialized automatically prior to first use; no action is required by the application.

In order to verify a server’s checksums, a client could simply compare the dictionaries using the equals method. However, this is not feasible if it is possible that the server might return a superset of the client’s checksums. A more general solution is to iterate over the local checksums as demonstrated below:

java.util.Map serverChecksums = ...
java.util.Iterator i = Checksums.checksums.entrySet().iterator();
while(i.hasNext()) {
    java.util.Map.Entry e = (java.util.Map.Entry)i.next();
    String id = (String)e.getKey();
    String checksum = (String)e.getValue();
    String serverChecksum = (String)serverChecksums.get(id);
    if (serverChecksum == null) {
        // No match found for type id!
    } else if (!checksum.equals(serverChecksum)) {
        // Checksum mismatch!
    }
}

In this example, the client first verifies that the server’s dictionary contains an entry for each Slice type ID, and then it proceeds to compare the checksums.