Can you explain this Spring environment variable resolution?

Question

The Spring Boot docs have the following sample logging file:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml" />
    <property name="LOG_FILE" value="${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}/}spring.log}"/>
    <include resource="org/springframework/boot/logging/logback/file-appender.xml" />
    <root level="INFO">
        <appender-ref ref="FILE" />
    </root>
</configuration>

Can you help me understand the line ${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}/}spring.log}? What are the - for?

Answer 1

Like Bash, Logback uses :- as its default value operator. The line in question is setting the LOG_FILE property:

• If LOG_FILE is already set, use that
• Otherwise, if LOG_PATH is set, use that suffixed with spring.log
• Otherwise, if LOG_TEMP is set, use that suffixed with /spring.log
• Otherwise, if java.io.tmpdir is set, use that suffixed with /spring.log
• Otherwise use /tmp/spring.log

Answer 2

It has nothing to do with Spring.

Logback XML configuration itself has such kind of placeholder handling to replace the placeholders with variable. The syntax for placeholder in logback is ${VARNAME}, and if you want default value if VARNAME is not present, you can do it by ${VARNAME:-DEFAULT} (ref: Logback Configuration). Yes, a :-, follow by the default value.

Then what you quote is easy to understand:

${LOG_FILE:-                                                             }
            ${LOG_PATH:-                                      }spring.log
                        ${LOG_TEMP:-                        }/
                                     ${java.io.tmpdir:-    }
                                                       /tmp

(Do you actually need explanation on what the above means?)