Fix #846 Formatter leaks threads and memory

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Fixes #847

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/google-java-format/pull/847 from stiemannkj1:fix-846-mem-thread-leak 0ca1e9bf9c8cf877509ac39cc50707d0ade7d71c
PiperOrigin-RevId: 507026969

core/src/main/java/com/google/googlejavaformat/java/Main.java

diff --git a/core/src/main/java/com/google/googlejavaformat/java/Main.java b/core/src/main/java/com/google/googlejavaformat/java/Main.java
index 11c0192..628c8bb 100644
--- a/core/src/main/java/com/google/googlejavaformat/java/Main.java
+++ b/core/src/main/java/com/google/googlejavaformat/java/Main.java
@@ -18,6 +18,7 @@
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.common.io.ByteStreams;
+import com.google.common.util.concurrent.MoreExecutors;
 import com.google.googlejavaformat.FormatterDiagnostic;
 import com.google.googlejavaformat.java.JavaFormatterOptions.Style;
 import java.io.IOError;
@@ -28,6 +29,7 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.time.Duration;
 import java.util.Arrays;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -187,6 +189,10 @@
         outWriter.write(formatted);
       }
     }
+    if (!MoreExecutors.shutdownAndAwaitTermination(executorService, Duration.ofSeconds(5))) {
+      errWriter.println("Failed to shut down ExecutorService");
+      allOk = false;
+    }
     return allOk ? 0 : 1;
   }