Behind the Fix: Adapting CDC %TOCHAR for Java 17’s GroupingSize Rules

Author By : @SHAILESH JAMLOKI  @KIRAN VENKATACHALA

A Journey from Java 8 to 17: Where CDC %TOCHAR Comes In

Upgrading a critical data replication product like IBM Data Replication (IDR) CDC from Java 8 to Java 17 is more than just switching to a new runtime—it’s about ensuring every subtle code path works seamlessly across environments. As part of our migration journey, we discovered an interesting behavior change in how Java 17 handles numeric formatting, specifically impacting CDC’s %TOCHAR function.

For those unfamiliar, %TOCHAR in CDC is used to convert numeric values, including floating-point and decimal types, into character strings for use in subscriptions and replication logic. It’s a workhorse function in many customer environments, especially when formatting or transforming numeric data during replication.

During our initial Java 17 testing, some subscriptions using %TOCHAR with decimal parameters began to fail. This wasn’t a compile-time problem—it was a runtime formatting issue that only surfaced under specific numeric conversion paths. While the failure didn’t occur everywhere, it pointed to a change in the way Java 17’s core libraries handled number formatting compared to Java 8.

This section of our migration story isn’t about a bug in CDC; it’s about how a small difference in a standard Java class taught us a big lesson about testing, compatibility, and paying attention to details when moving across LTS versions.

When 1000 Becomes Too Big: The New GroupingSize Rule

At the heart of this issue was Java’s DecimalFormat class—the same class that controls how numbers are turned into formatted strings. Inside CDC’s %TOCHAR implementation, DecimalFormat.setGroupingSize() is used to control the number of digits between grouping separators. For example, with a grouping size of 3, the number 123456.78 is formatted as 123,456.78.

In CDC’s code, the grouping size was intentionally set to a very large value (1000) to effectively disable grouping while keeping other formatting consistent. This approach worked perfectly fine in Java 8 because setGroupingSize() accepted any positive integer.

Java 17, however, introduced a subtle but important restriction. The new implementation now limits the grouping size to the range of a byte (0–127). Internally, the value you pass is converted to a byte, and if it’s outside the allowed range, Java throws an exception:

java.lang.IllegalArgumentException: newValue is out of valid range. value: 1000

This change is explicitly mentioned in the Java 17 API docs:

“The value passed in is converted to a byte, which may lose information. Values that are negative or greater than Byte.MAX_VALUE will throw an IllegalArgumentException.”
— Java 17 DecimalFormat API

This meant that when CDC attempted to set the grouping size to 1000 under Java 17, it immediately triggered this exception, breaking subscriptions that used %TOCHAR with a decimal parameter. The difference wasn’t in CDC’s logic, but in the stricter boundary check introduced in Java 17, turning what had been a safe “disable grouping” approach into an invalid value.

This was the moment we realized: a simple 1000 had just become “too big."

Behind the Fix: Teaching %TOCHAR to Play by Java 17’s Rules

Once we traced the failure back to DecimalFormat.setGroupingSize(1000), the solution became clear: %TOCHAR simply needed to comply with Java 17’s new boundary. Instead of using a hard-coded value of 1000 to disable grouping, we aligned it to the maximum valid limit—Byte.MAX_VALUE (127).

Here’s what the change looked like inside CDC’s %TOCHAR implementation:

DecimalFormat df = new DecimalFormat();

// Before Java 17 migration:
df.setGroupingSize(1000);

// Updated for Java 17 compatibility:
df.setGroupingSize(Byte.MAX_VALUE);

df.setMinimumFractionDigits(decimals);
df.setMaximumFractionDigits(decimals);

By setting the grouping size to Byte.MAX_VALUE, the formatter still behaves effectively the same way as the old code in practical scenarios, but now fully adheres to Java 17’s rules.

During testing, we also provided a temporary workaround for customers already in the middle of migration: removing the optional decimal parameter from %TOCHAR.

Workaround example:

• Before (with decimal parameter):

  • %REPLACE((%TOCHAR(DECIMAL_COL,10,2)), "*ALL", ".", "")
    

• After (without decimal parameter):
  • %REPLACE((%TOCHAR(DECIMAL_COL,10)), "*ALL", ".", "")

It’s worth noting that dropping the decimal parameter changes the behavior of %TOCHAR. For instance, with an input value of 1.23:

• With decimal parameter: 000000123

• Without decimal parameter: 123000000

While this workaround helped unblock migrations in progress, the permanent fix in CDC ensures customers don’t need to change expressions at all. %TOCHAR now handles both Java 8 and Java 17 runtimes gracefully, making the upgrade seamless going forward.

Beyond the Fix: Key Insights from the Migration

When we first hit the setGroupingSize(1000) failure on Java 17, it highlighted an important reality of major runtime upgrades: some differences only show up under real workloads. This wasn’t a code change in CDC itself—it was a subtle shift in the JDK’s behavior that only surfaced when %TOCHAR with a decimal parameter hit the new boundary.

The real insight wasn’t about scanning every API in advance—that’s not practical for a product of CDC’s size. Instead, it came down to:

• Running realistic scenarios early. The issue appeared only when subscriptions invoked %TOCHAR with decimals, which made test coverage critical.

• Tracing failures to the source. Quickly isolating the problem to Java’s DecimalFormat helped us move from symptom to root cause.

• Designing a future-proof fix. Using Byte.MAX_VALUE ensures the function behaves consistently on both Java 8 and Java 17 without customers needing to change expressions.

In the end, the change itself was a single line of code, but it underscored the value of adaptive testing and quick diagnosis during LTS migrations. Rather than a sweeping rewrite, this was a reminder that smooth upgrades often come down to catching—and resolving—the smallest details.