Support persisting class-name filters

Author: mcculls

utils/src/main/java/datadog/instrument/utils/ClassNameFilter.java

@@ -6,6 +6,9 @@
 
 package datadog.instrument.utils;
 
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
 import java.util.Arrays;
 
 /**
@@ -23,6 +26,8 @@ public final class ClassNameFilter {
   private static final int MIN_CAPACITY = 1 << 8;
   private static final int MAX_HASH_ATTEMPTS = 10;
 
+  private static final int FILE_MAGIC = 0xDD094172;
+
   // fixed-size hashtable of encoded members, indexed by class-name
   private final long[] members;
   private final int slotMask;
@@ -43,6 +48,11 @@ public ClassNameFilter(int capacity) {
     members = new long[slotMask + 1];
   }
 
+  private ClassNameFilter(int slotMask, long[] members) {
+    this.slotMask = slotMask;
+    this.members = members;
+  }
+
   /**
    * Tests whether the given class-name is a member of the filter.
    *
@@ -106,6 +116,40 @@ public void clear() {
     Arrays.fill(members, 0);
   }
 
+  /**
+   * Writes filter content to an external resource.
+   *
+   * @param out where to write the serialized filter
+   * @throws IOException if the content cannot be written
+   */
+  public void writeTo(DataOutput out) throws IOException {
+    out.writeInt(FILE_MAGIC);
+    out.writeInt(slotMask);
+    for (long m : members) {
+      out.writeLong(m);
+    }
+  }
+
+  /**
+   * Reads filter content from an external resource.
+   *
+   * @param in the serialized filter content
+   * @return deserialized class-name filter
+   * @throws IOException if the content cannot be read
+   */
+  public static ClassNameFilter readFrom(DataInput in) throws IOException {
+    int magic = in.readInt();
+    if (magic != FILE_MAGIC) {
+      throw new IOException("Unexpected file magic " + magic);
+    }
+    int slotMask = in.readInt();
+    long[] members = new long[slotMask + 1];
+    for (int i = 0; i < members.length; i++) {
+      members[i] = in.readLong();
+    }
+    return new ClassNameFilter(slotMask, members);
+  }
+
   /**
    * Computes a 32-bit 'class-code' that includes the length of the package-prefix and simple-name,
    * plus the first and last characters of the simple-name (each truncated to fit into 8-bits.)

utils/src/test/java/datadog/instrument/utils/ClassNameFilterTest.java

@@ -5,6 +5,13 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.UncheckedIOException;
 import java.util.HashSet;
 import java.util.Set;
 import org.junit.jupiter.api.Test;
@@ -74,4 +81,27 @@ void overflow() {
       }
     }
   }
+
+  @Test
+  void roundTrip() {
+    ClassNameFilter exporter = new ClassNameFilter(8);
+
+    exporter.add("example.MyClass");
+    exporter.add("example.test.FB");
+    exporter.add("example.test.Ea");
+
+    ClassNameFilter importer;
+    try (ByteArrayOutputStream sink = new ByteArrayOutputStream()) {
+      exporter.writeTo(new DataOutputStream(sink));
+      try (InputStream source = new ByteArrayInputStream(sink.toByteArray())) {
+        importer = ClassNameFilter.readFrom(new DataInputStream(source));
+      }
+    } catch (IOException e) {
+      throw new UncheckedIOException(e);
+    }
+
+    assertTrue(importer.contains("example.MyClass"));
+    assertTrue(importer.contains("example.test.FB"));
+    assertFalse(importer.contains("example.test.Ea"));
+  }
 }