Rely more on Maven in updaterelease.sh:

- Use Maven Wrapper. This builds on cl/573917287.
- Derive the location of Java 11 from the Maven build instead of requiring `JAVA_HOME` to be set. This might let us choose to stop requiring `JAVA_HOME`. See cl/639128020 and related CLs.

updaterelease.sh

@@ -94,6 +94,7 @@ remove_unnecessary_comments() {
 # Generates Javadoc for the given flavor of Guava.
 generate_javadoc() {
   local flavor="$1"
+  local mvnw="$PWD/mvnw"
 
   # Change to android directory if necessary.
   if [[ "$flavor" == android ]]; then
@@ -117,7 +118,7 @@ generate_javadoc() {
   # Compile and generate Javadoc.
   # Save the build classpath to a file in the tempdir.
   echo -n "Compiling and generating Javadoc for Guava $version..."
-  mvn \
+  "${mvnw}" \
     clean \
     compile \
     javadoc:javadoc \
@@ -153,7 +154,7 @@ generate_jdiff_xml() {
 
   # Generate JDiff XML file for the release.
   echo -n "Generating JDiff XML for Guava $version..."
-  ${JAVA_HOME}/bin/javadoc \
+  ${JAVA_11_HOME}/bin/javadoc \
     -sourcepath "$TEMPDIR/$flavor/src" \
     -classpath "$classpath" \
     -subpackages com.google.common \
@@ -202,7 +203,7 @@ jdiff() {
   local prev_release_javadoc_path="../../../../$other/api/docs/"
 
   echo -n "Generating JDiff report between Guava $other_version and $version..."
-  ${JAVA_HOME}/bin/javadoc \
+  ${JAVA_11_HOME}/bin/javadoc \
     -subpackages com \
     -doclet jdiff.JDiff \
     -docletpath "$JDIFF_PATH" \
@@ -388,14 +389,6 @@ generate_snapshot_javadoc_shortlinks() {
 
 # Main function actually run the process.
 main() {
-  if [[ -z "${JAVA_HOME:-}" ]]; then
-    echo '$JAVA_HOME needs to be set' >&2
-    exit 1
-  elif [[ ! -x "${JAVA_HOME}/bin/javadoc" ]]; then
-    echo '$JAVA_HOME must be set to a valid JDK location but is '"${JAVA_HOME}" >&2
-    exit 1
-  fi
-
   # Make sure we have all the latest tags
   git fetch --tags
 
@@ -416,6 +409,11 @@ main() {
   #   classes/
   #   docs/
   git_checkout_ref "$RELEASE_REF"
+
+  # We run this separately so that its change to the default toolchain doesn't affect anything else.
+  ./mvnw --projects '!guava-testlib,!guava-tests,!guava-bom,!guava-gwt' initialize -P print-java-11-home
+  export JAVA_11_HOME=$(<target/java_11_home)
+
   for flavor in "${FLAVORS[@]}"; do
     generate_javadoc "$flavor"
   done