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Add property expansion example
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‎src/site/antora/modules/ROOT/examples/manual/configuration/loggers.properties

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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to you under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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##
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# tag::loggers[]
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rootLogger.level = INFO # <1>
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rootLogger.appenderRef.0.ref = APPENDER1
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@@ -11,4 +29,5 @@ logger.1.appenderRef.0.ref = APPENDER3
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logger.2.name = org.example.level # <4>
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logger.2.level = DEBUG
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logger.2.appenderRef.0.ref = APPENDER4
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logger.2.appenderRef.0.ref = APPENDER4
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# end::loggers[]

‎src/site/antora/modules/ROOT/examples/manual/configuration/loggers.xml

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<?xml version="1.0" encoding="UTF-8" ?>
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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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~ Licensed to the Apache Software Foundation (ASF) under one or more
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~ contributor license agreements. See the NOTICE file distributed with
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~ this work for additional information regarding copyright ownership.
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~ The ASF licenses this file to you under the Apache License, Version 2.0
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~ (the "License"); you may not use this file except in compliance with
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~ the License. You may obtain a copy of the License at
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~
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~ http://www.apache.org/licenses/LICENSE-2.0
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~
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~ Unless required by applicable law or agreed to in writing, software
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~ distributed under the License is distributed on an "AS IS" BASIS,
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~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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~ See the License for the specific language governing permissions and
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~ limitations under the License.
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-->
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<Configuration xmlns="https://logging.apache.org/xml/ns"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="
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</Logger>
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</Loggers>
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<!-- end::loggers[] -->
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</Configuration>
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</Configuration>

‎src/site/antora/modules/ROOT/examples/manual/configuration/loggers.yaml

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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to you under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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Configuration:
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# tag::loggers[]
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Loggers:
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level: "DEBUG"
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AppenderRef:
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ref: "APPENDER4"
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# end::loggers[]
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# end::loggers[]

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-example.json

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{
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"Properties": {
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"Property": [
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{
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"name": "FOO",
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"value": "foo"
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},
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{
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"name": "BAR",
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"value": "bar"
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}
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]
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}
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}

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-example.properties

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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to you under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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property.FOO = foo
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property.BAR = bar

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-example.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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~ Licensed to the Apache Software Foundation (ASF) under one or more
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~ contributor license agreements. See the NOTICE file distributed with
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~ this work for additional information regarding copyright ownership.
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~ The ASF licenses this file to you under the Apache License, Version 2.0
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~ (the "License"); you may not use this file except in compliance with
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~ the License. You may obtain a copy of the License at
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~
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~ http://www.apache.org/licenses/LICENSE-2.0
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~
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~ Unless required by applicable law or agreed to in writing, software
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~ distributed under the License is distributed on an "AS IS" BASIS,
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~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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~ See the License for the specific language governing permissions and
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~ limitations under the License.
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-->
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<Properties>
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<Property name="FOO" value="foo"/>
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<Property name="BAR" value="bar"/>
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</Properties>

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-example.yaml

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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to you under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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Properties:
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Property:
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- name: "FOO"
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value: "foo"
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- name: "BAR"
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value: "bar"

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-recursion.json

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{
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"Properties": {
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"Property": [
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{
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"name": "logging.file",
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"value": "${logging.path}/app.log"
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},
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{
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"name": "logging.dir",
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"value": "${env:APP_BASE}/logs"
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},
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{
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"name": "APP_BASE",
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"value": "."
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}
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]
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}
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}

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-recursion.properties

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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to you under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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property.logging.file = ${logging.dir}/app.log
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property.logging.dir = ${env:APP_BASE}/logs
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property.APP_BASE = .

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-recursion.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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~ Licensed to the Apache Software Foundation (ASF) under one or more
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~ contributor license agreements. See the NOTICE file distributed with
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~ this work for additional information regarding copyright ownership.
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~ The ASF licenses this file to you under the Apache License, Version 2.0
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~ (the "License"); you may not use this file except in compliance with
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~ the License. You may obtain a copy of the License at
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~
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~ http://www.apache.org/licenses/LICENSE-2.0
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~
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~ Unless required by applicable law or agreed to in writing, software
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~ distributed under the License is distributed on an "AS IS" BASIS,
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~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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~ See the License for the specific language governing permissions and
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~ limitations under the License.
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-->
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<Properties>
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<Property name="logging.file" value="${logging.dir}/app.log"/>
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<Property name="logging.dir" value="${env:APP_BASE}/logs"/>
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<Property name="APP_BASE" value="."/>
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</Properties>

‎src/site/antora/modules/ROOT/examples/manual/configuration/properties-recursion.yaml

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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to you under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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Properties:
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Property:
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- name: "logging.file"
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value: "${logging.dir}/app.log"
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- name: "logging.dir"
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value: "${env:APP_BASE}/logs"
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- name: "APP_BASE"
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value: "."

‎src/site/antora/modules/ROOT/pages/manual/configuration.adoc

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[source,properties]
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----
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include::example$manual/configuration/loggers.properties[]
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include::example$manual/configuration/loggers.properties[tags=loggers]
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----
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====
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The same rule applies to the `name` parameter: if it contains a `${` sequence, the sequence must be escaped as `$${`.
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====
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.Property substitution example
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=====
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If your configuration file contains the following definitions:
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[tabs]
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====
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XML::
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+
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[source,xml]
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----
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include::example$manual/configuration/properties-example.xml[lines=18..-1]
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----
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JSON::
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+
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[source,json]
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----
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include::example$manual/configuration/properties-example.json[]
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----
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YAML::
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+
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[source,yaml]
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----
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include::example$manual/configuration/properties-example.yaml[lines=17..-1]
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----
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Java properties::
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[source,properties]
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----
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include::example$manual/configuration/properties-example.properties[lines=17..-1]
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----
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====
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and the OS environment variable `FOO` has a value of `environment`, Log4j will evaluate expression as follows
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[cols="1m,1m"]
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|===
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| Expression | Value
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| $\{FOO} | foo
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| $\{BAZ} | $\{BAZ}
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| ${BAR:-$\{FOO}} | bar
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| ${BAZ:-$\{FOO}} | foo
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| ${env:FOO} | environment
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| ${env:BAR} | bar
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| ${env:BAZ} | $\{BAZ}
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| ${env:BAR:-$\{FOO}} | bar
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| ${env:BAZ:-$\{FOO}} | foo
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|===
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=====
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[CAUTION]
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=====
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For security reasons, if the **expansion** of a `${...}` expression contains other expressions, these will **not** be expanded.
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The only exception to this rule is the expansion of properties in the `Properties` container.
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Properties defined in the `Properties` container can depend on each other.
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If your configuration contains, for example:
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[tabs]
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====
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XML::
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+
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[source,xml]
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----
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include::example$manual/configuration/properties-recursion.xml[lines=18..-1]
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----
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JSON::
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+
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[source,json]
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----
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include::example$manual/configuration/properties-recursion.json[]
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----
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YAML::
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+
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[source,yaml]
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----
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include::example$manual/configuration/properties-recursion.yaml[lines=17..-1]
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----
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Java properties::
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+
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[source,properties]
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----
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include::example$manual/configuration/properties-recursion.properties[lines=17..-1]
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----
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====
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the `logging.dir` property will be expanded **before** the `logging.file` property and the expanded value will be substituted in `${logging.dir}/app.log`.
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Therefore, the value of the `logging.file` property will be:
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* `./logs/app.log` if the environment variable `APP_BASE` is not defined,
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* `/var/lib/app/logs/app.log` if the environment variable `APP_BASE` has a value of `/var/lib/app`.
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=====
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[id=lazy-property-substitution]
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==== Lazy property substitution
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