Fix Thymeleaf null boolean error (SpEL issue)

Problem: Thymeleaf null boolean error

You may encounter this error when rendering a Thymeleaf template:

cannot convert from null to boolean

This often appears only for specific users, such as test accounts or newly created data.

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Environment

• Spring Boot
• Thymeleaf
• Spring Expression Language (SpEL)

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Root Cause

The issue comes from how SpEL evaluates logical expressions.

Consider this code:

th:value="${cmpyNo == null or isWonmaster ? '' : cmpyNo}"

If isWonmaster is null, the expression becomes:

true OR null

SpEL does not allow null in logical operations. Both operands must be boolean.

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Solution

1. Make the expression null-safe

th:value="${cmpyNo == null or isWonmaster == true ? '' : cmpyNo}"

or

th:value="${cmpyNo == null or (isWonmaster ?: false) ? '' : cmpyNo}"

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2. Fix it at the Controller level (recommended)

Normalize the value before passing it to the view:

model.addAttribute("isWonmaster",
Boolean.TRUE.equals(user.getIsWonmaster()));

Then your template can safely use:

th:value="${cmpyNo == null or isWonmaster ? '' : cmpyNo}"

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Why it only happens sometimes

This bug only occurs when the value is null:

|Value|Result| |---|---| |true|OK| |false|OK| |null|ERROR|

That’s why it often slips through testing and appears in edge cases.

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Best Practice

• Always assume Boolean can be null
• Never use it directly in logical expressions
• Normalize it before evaluation

Recommended patterns:

isFlag == true
isFlag ?: false

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Takeaway

This is not a Thymeleaf bug but a SpEL design constraint.

The most robust solution is:

• eliminate null at the Controller layer
• keep templates simple and predictable