Target Java 8:

- Set `-source 8 -target 8`. We already have a depencdency on `guava-android`, which began requiring Java 8 a while back, so we might as well unlock the ability to use Java 8 language features ourselves. (I also found I had to set `-source 8` in `maven-javadoc-plugin` to prevent it from trying to pass a `--module-path`, which leads to problems with our usages of `@NullableDecl`, which isn't in a module. That's probably unrelated: We probably just haven't run Javadoc since some `maven-javadoc-plugin` upgrade.) Work toward #240, and fixes #229
- Don't refer to Java 9+ `ByteBuffer` methods, and set up Error Prone enforcement of future such mistakes. Compare google/guava#6334. Fixes #113

RELNOTES=Officially dropped support for Java 7, and restored accidentally dropped support for Java 8.
PiperOrigin-RevId: 536468340

README.md

@@ -1,7 +1,7 @@
 Jimfs
 =====
 
-Jimfs is an in-memory file system for Java 7 and above, implementing the
+Jimfs is an in-memory file system for Java 8 and above, implementing the
 [java.nio.file](http://docs.oracle.com/javase/7/docs/api/java/nio/file/package-summary.html)
 abstract file system APIs.
 

jimfs/src/main/java/com/google/common/jimfs/Java8Compatibility.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2020 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.google.common.jimfs;
+
+import java.nio.Buffer;
+
+/**
+ * Wrappers around {@link Buffer} methods that are covariantly overridden in Java 9+. See
+ * https://github.com/google/guava/issues/3990
+ */
+final class Java8Compatibility {
+  static void clear(Buffer b) {
+    b.clear();
+  }
+
+  private Java8Compatibility() {}
+}

jimfs/src/main/java/com/google/common/jimfs/RegularFile.java

@@ -579,7 +579,7 @@ public long transferTo(long pos, long count, WritableByteChannel dest) throws IO
       while (buf.hasRemaining()) {
         remaining -= dest.write(buf);
       }
-      buf.clear();
+      Java8Compatibility.clear(buf);
 
       while (remaining > 0) {
         int index = ++blockIndex;
@@ -589,7 +589,7 @@ public long transferTo(long pos, long count, WritableByteChannel dest) throws IO
         while (buf.hasRemaining()) {
           remaining -= dest.write(buf);
         }
-        buf.clear();
+        Java8Compatibility.clear(buf);
       }
     }
 

pom.xml

@@ -96,7 +96,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <auto-service.version>1.0.1</auto-service.version>
     <auto-common.version>1.2.1</auto-common.version>
-    <java.version>1.7</java.version>
+    <java.version>1.8</java.version>
     <guava.version>31.1-android</guava.version>
     <!--
       NOTE: When updating errorprone.version, also update javac.version to the
@@ -182,6 +182,7 @@
             <encoding>UTF-8</encoding>
             <docencoding>UTF-8</docencoding>
             <charset>UTF-8</charset>
+            <source>${java.version}</source>
             <detectJavaApiLink>false</detectJavaApiLink>
             <links>
               <link>https://checkerframework.org/api/</link>
@@ -304,7 +305,7 @@
               <compilerArgs>
                 <!-- https://errorprone.info/docs/installation#maven -->
                 <arg>-XDcompilePolicy=simple</arg>
-                <arg>-Xplugin:ErrorProne</arg>
+                <arg>-Xplugin:ErrorProne -Xep:Java8ApiChecker:ERROR</arg>
                 <!-- https://github.com/google/error-prone/blob/f8e33bc460be82ab22256a7ef8b979d7a2cacaba/docs/installation.md#jdk-16 -->
                 <!-- TODO(cpovirk): Use .mvn/jvm.config instead (per
                      https://errorprone.info/docs/installation#maven) if it can