Fix confusing build failure when javac is missing

README.md

@@ -69,11 +69,16 @@ To build PXF, you must have:
     source /usr/local/cloudberry-db/cloudberry-env.sh # For Cloudberry 2.1+
     ```
 
-3. JDK 1.8 or JDK 11 to compile/run
+3. A full JDK -- 1.8, 11, 17, or 21 -- to compile/run. A JRE is not enough, as the
+   server module needs `javac`.
+
+    > [!NOTE]
+    > Support for JDK 1.8 will be removed in PXF 3.0. Use JDK 11 or later for new setups.
 
     Export your `JAVA_HOME`:
     ```
-    export JAVA_HOME=/usr/lib/jvm/java-11-openjdk
+    export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64  # Debian/Ubuntu
+    export JAVA_HOME=/usr/lib/jvm/java-11-openjdk        # RHEL/Rocky
     ```
 
 4. Go (1.25 or later)
@@ -96,6 +101,8 @@ To build PXF, you must have:
 
 PXF uses Makefiles to build its components. PXF server component uses Gradle that is wrapped into the Makefile for convenience.
 
+If the build fails, see [TROUBLESHOOTING.md](TROUBLESHOOTING.md#build-issues).
+
 > [!NOTE]
 > To comply with Apache Software Foundation release guidelines, `gradle-wrapper.jar` is not included in the source distribution. It will be downloaded automatically during the initial build. Please ensure you have an active and stable internet connection.
 
@@ -163,7 +170,7 @@ We provide a Docker-based development environment that includes Cloudberry, Hado
 
 - Start IntelliJ. Click "Open" and select the directory to which you cloned the `pxf` repo.
 - Select `File > Project Structure`.
-- Make sure you have a JDK (version 1.8) selected.
+- Make sure you have a JDK selected (1.8, 11, 17, or 21). CI builds with JDK 11 by default.
 - In the `Project Settings > Modules` section, select `Import Module`, pick the `pxf/server` directory and import as a Gradle module. You may see an error saying that there's
 no JDK set for Gradle. Just cancel and retry. It goes away the second time.
 - Import a second module, giving the `pxf/automation` directory, select "Import module from external model", pick `Maven` then click Finish.

TROUBLESHOOTING.md

@@ -1,5 +1,46 @@
 # Troubleshooting
 
+## Build Issues
+
+### Gradle reports `does not provide the required capabilities: [JAVA_COMPILER]`
+
+```
+Execution failed for task ':pxf-api:compileJava'.
+> Error while evaluating property 'javaCompiler' of task ':pxf-api:compileJava'.
+   > Toolchain installation '/usr/lib/jvm/java-11-openjdk-amd64' does not provide
+     the required capabilities: [JAVA_COMPILER]
+```
+
+The JVM Gradle is running on has no `javac`, so it is a JRE rather than a JDK.
+Gradle itself starts fine on a JRE, which is why the build gets as far as
+`:pxf-api:compileJava` before failing. Install a JDK and point `JAVA_HOME` at it:
+
+```
+# Debian/Ubuntu
+sudo apt-get install -y openjdk-11-jdk
+export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
+
+# RHEL/Rocky
+sudo dnf install -y java-11-openjdk-devel
+export JAVA_HOME=/usr/lib/jvm/java-11-openjdk
+```
+
+Two things make this easy to misdiagnose:
+
+* On Debian/Ubuntu the JRE is installed into `/usr/lib/jvm/java-11-openjdk-amd64`,
+  the same directory a JDK would use, so `java -version` and `ls /usr/lib/jvm` both
+  look healthy while `javac` is absent. The `maven` package depends on
+  `default-jre-headless` and does **not** pull in a JDK. (On RHEL/Rocky,
+  `dnf install maven` does install one, via `maven-jdk-binding`.)
+* Installing the JDK is not sufficient on its own. The Gradle daemon caches JVM
+  installation metadata for its whole lifetime and never re-checks the filesystem,
+  and because the JDK lands in the same *path*, Gradle reuses the daemon left behind
+  by the failed build and reports the identical error. Stop it first:
+
+  ```
+  cd server && ./gradlew --stop
+  ```
+
 ## Out of Memory Issues
 
 ### 

server/Makefile

@@ -27,8 +27,31 @@ PXF_API_VERSION ?= $(shell cat $(PXF_ROOT_DIR)/api_version)
 
 PXF_GRADLE_PROPERTIES = -Pversion=$(PXF_VERSION) -PapiVersion=$(PXF_API_VERSION)
 
+# Gradle can start on a JRE, but compiling PXF needs a JDK. Detect a JRE-only
+# installation here so the build fails with an actionable message instead of
+# Gradle's "does not provide the required capabilities: [JAVA_COMPILER]".
+JAVAC := $(if $(JAVA_HOME),$(JAVA_HOME)/bin/javac,$(shell command -v javac 2>/dev/null))
+
+.PHONY: check-jdk
+check-jdk:
+	@if [ ! -x "$(JAVAC)" ]; then \
+		echo "ERROR: no Java compiler (javac) found -- a JDK is required to build PXF, a JRE is not enough."; \
+		if [ -n "$(JAVA_HOME)" ]; then \
+			echo "       JAVA_HOME=$(JAVA_HOME) has no bin/javac, so it points at a JRE."; \
+		else \
+			echo "       JAVA_HOME is not set and javac is not on PATH."; \
+		fi; \
+		echo "       Install a JDK and point JAVA_HOME at it, for example:"; \
+		echo "         Debian/Ubuntu: sudo apt-get install -y openjdk-11-jdk"; \
+		echo "         RHEL/Rocky:    sudo dnf install -y java-11-openjdk-devel"; \
+		echo "         export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64"; \
+		echo "       If a build already failed here, also run './gradlew --stop'."; \
+		echo "       See TROUBLESHOOTING.md for details."; \
+		exit 2; \
+	fi
+
 .PHONY: prepare-gradle-wrapper
-prepare-gradle-wrapper:
+prepare-gradle-wrapper: check-jdk
 	@APP_HOME="$(CURDIR)" bash ./gradlew-install.sh
 
 help: