Add flags module

FileFlagSource reads flags from files in /etc/vespa/flags and is a component
that can be injected in host admin, config server, etc. A flag named foo
corresponds to filename foo.

In general a FlagSource manages:
 - Feature flags: A feature is either set (true/enabled) or otherwise false.
   Touching a file foo means the feature flag foo is set (true).
 - Value flags: Either a String or empty if not set. The String corresponds to
   the file content.

The plan is to make the config server another source of flags. A unified
FlagSource can merge the two sources with some priority and used in e.g. parts
of node-admin. In other parts one would only have access to the file source.

Defines various flag facades:
 - FeatureFlag: Used to test whether a feature has been enabled or not.
 - IntFlag
 - JacksonFlag: Deserializes JSON to Jackson class, or return default if unset.
 - LongFlag
 - OptionalJacksonFlag: Deserializes JSON to Jackson class, or empty if unset.
 - OptionalStringFlag
 - StringFlag

This is part of removing some of the last Chef recipes. Some minor tweaks have
been necessary as part of this and are included in this PR (test whether a
systemd service exists, task-friendly file deletion, allow capitalized letters
in YUM package name).

15 files changed, 691 insertions, 0 deletions

diff --git a/flags/pom.xml b/flags/pom.xml
new file mode 100644
index 00000000000..57c57f907d3
--- /dev/null
+++ b/flags/pom.xml
@@ -0,0 +1,66 @@
+<?xml version="1.0"?>
+<!-- Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root. -->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
+                             http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.yahoo.vespa</groupId>
+        <artifactId>parent</artifactId>
+        <version>6-SNAPSHOT</version>
+        <relativePath>../parent/pom.xml</relativePath>
+    </parent>
+
+    <artifactId>flags</artifactId>
+    <version>6-SNAPSHOT</version>
+    <packaging>container-plugin</packaging>
+    <name>${project.artifactId}</name>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.yahoo.vespa</groupId>
+            <artifactId>defaults</artifactId>
+            <version>${project.version}</version>
+            <scope>provided</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.yahoo.vespa</groupId>
+            <artifactId>container-dev</artifactId>
+            <version>${project.version}</version>
+            <scope>provided</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.hamcrest</groupId>
+            <artifactId>hamcrest-junit</artifactId>
+            <version>2.0.0.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.yahoo.vespa</groupId>
+            <artifactId>testutil</artifactId>
+            <version>${project.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>com.yahoo.vespa</groupId>
+                <artifactId>bundle-plugin</artifactId>
+                <extensions>true</extensions>
+            </plugin>
+        </plugins>
+    </build>
+</project>
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/FeatureFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/FeatureFlag.java
new file mode 100644
index 00000000000..957faee41cc
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/FeatureFlag.java
@@ -0,0 +1,48 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import java.util.function.Function;
+
+/**
+ * A FeatureFlag is true only if set (enabled).
+ *
+ * @author hakonhall
+ */
+public class FeatureFlag implements Flag {
+    private final FlagId id;
+    private final FlagSource source;
+
+    public static Function<FlagSource, FeatureFlag> createUnbound(String flagId) {
+        return createUnbound(new FlagId(flagId));
+    }
+
+    public static Function<FlagSource, FeatureFlag> createUnbound(FlagId flagId) {
+        return source -> new FeatureFlag(flagId, source);
+    }
+
+    public FeatureFlag(String flagId, FlagSource source) {
+        this(new FlagId(flagId), source);
+    }
+
+    public FeatureFlag(FlagId id, FlagSource source) {
+        this.id = id;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public boolean isSet() {
+        return source.hasFeature(id);
+    }
+
+    @Override
+    public String toString() {
+        return "FeatureFlag{" +
+                "id=" + id +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/FileFlagSource.java b/flags/src/main/java/com/yahoo/vespa/flags/FileFlagSource.java
new file mode 100644
index 00000000000..f13a122b156
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/FileFlagSource.java
@@ -0,0 +1,67 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import com.google.common.util.concurrent.UncheckedTimeoutException;
+
+import javax.inject.Inject;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.FileSystem;
+import java.nio.file.FileSystems;
+import java.nio.file.Files;
+import java.nio.file.NoSuchFileException;
+import java.nio.file.Path;
+import java.util.Optional;
+
+/**
+ * A {@link FlagSource} backed by local files.
+ *
+ * @author hakonhall
+ */
+public class FileFlagSource implements FlagSource {
+    static final String FLAGS_DIRECTORY = "/etc/vespa/flags";
+
+    private final Path flagsDirectory;
+
+    @Inject
+    public FileFlagSource() {
+        this(FileSystems.getDefault());
+    }
+
+    public FileFlagSource(FileSystem fileSystem) {
+        this(fileSystem.getPath(FLAGS_DIRECTORY));
+    }
+
+    public FileFlagSource(Path flagsDirectory) {
+        this.flagsDirectory = flagsDirectory;
+    }
+
+    @Override
+    public boolean hasFeature(FlagId id) {
+        return Files.exists(getPath(id));
+    }
+
+    @Override
+    public Optional<String> getString(FlagId id) {
+        return getBytes(id).map(bytes -> new String(bytes, StandardCharsets.UTF_8));
+    }
+
+    public Optional<byte[]> getBytes(FlagId id) {
+        try {
+            return Optional.of(Files.readAllBytes(getPath(id)));
+        } catch (NoSuchFileException e) {
+            return Optional.empty();
+        } catch (IOException e) {
+            throw new UncheckedTimeoutException(e);
+        }
+    }
+
+    private Path getPath(FlagId id) {
+        return flagsDirectory.resolve(id.toString());
+    }
+
+    @Override
+    public String toString() {
+        return "FileFlagSource{" + flagsDirectory + '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/Flag.java b/flags/src/main/java/com/yahoo/vespa/flags/Flag.java
new file mode 100644
index 00000000000..831e0d0dab9
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/Flag.java
@@ -0,0 +1,9 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+/**
+ * @author hakonhall
+ */
+public interface Flag {
+    FlagId id();
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/FlagId.java b/flags/src/main/java/com/yahoo/vespa/flags/FlagId.java
new file mode 100644
index 00000000000..f004df063ed
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/FlagId.java
@@ -0,0 +1,42 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import javax.annotation.concurrent.Immutable;
+import java.util.Objects;
+import java.util.regex.Pattern;
+
+/**
+ * @author hakonhall
+ */
+@Immutable
+public class FlagId {
+    private static final Pattern ID_PATTERN = Pattern.compile("^[a-zA-Z0-9][a-zA-Z0-9._-]*$");
+
+    private final String id;
+
+    public FlagId(String id) {
+        if (!ID_PATTERN.matcher(id).find()) {
+            throw new IllegalArgumentException("Not a valid FlagId: '" + id + "'");
+        }
+
+        this.id = id;
+    }
+
+    @Override
+    public String toString() {
+        return id;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        FlagId flagId = (FlagId) o;
+        return Objects.equals(id, flagId.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/FlagSource.java b/flags/src/main/java/com/yahoo/vespa/flags/FlagSource.java
new file mode 100644
index 00000000000..56f4b5ee0ae
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/FlagSource.java
@@ -0,0 +1,15 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import java.util.Optional;
+
+/**
+ * @author hakonhall
+ */
+public interface FlagSource {
+    /** Whether the source has the feature flag with the given id. */
+    boolean hasFeature(FlagId id);
+
+    /** The String value of a flag. */
+    Optional<String> getString(FlagId id);
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/IntFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/IntFlag.java
new file mode 100644
index 00000000000..f7c9645c5db
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/IntFlag.java
@@ -0,0 +1,49 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import java.util.function.Function;
+
+/**
+ * @author hakonhall
+ */
+public class IntFlag implements Flag {
+    private final FlagId id;
+    private final int defaultValue;
+    private final FlagSource source;
+
+    public static Function<FlagSource, IntFlag> createUnbound(String flagId, int defaultValue) {
+        return createUnbound(new FlagId(flagId), defaultValue);
+    }
+
+    public static Function<FlagSource, IntFlag> createUnbound(FlagId id, int defaultValue) {
+        return source -> new IntFlag(id, defaultValue, source);
+    }
+
+    public IntFlag(String flagId, int defaultValue, FlagSource source) {
+        this(new FlagId(flagId), defaultValue, source);
+    }
+
+    public IntFlag(FlagId id, int defaultValue, FlagSource source) {
+        this.id = id;
+        this.defaultValue = defaultValue;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public int value() {
+        return source.getString(id).map(String::trim).map(Integer::parseInt).orElse(defaultValue);
+    }
+
+    @Override
+    public String toString() {
+        return "IntFlag{" +
+                "id=" + id +
+                ", defaultValue=" + defaultValue +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/JacksonFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/JacksonFlag.java
new file mode 100644
index 00000000000..99add358e75
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/JacksonFlag.java
@@ -0,0 +1,58 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import java.util.function.Function;
+
+import static com.yahoo.yolean.Exceptions.uncheck;
+
+/**
+ * @author hakonhall
+ */
+public class JacksonFlag<T> implements Flag {
+    private final static ObjectMapper mapper = new ObjectMapper();
+
+    private final FlagId id;
+    private final Class<T> jacksonClass;
+    private final T defaultValue;
+    private final FlagSource source;
+
+    public static <T> Function<FlagSource, JacksonFlag<T>> createUnbound(String flagId, Class<T> jacksonClass, T defaultValue) {
+        return createUnbound(new FlagId(flagId), jacksonClass, defaultValue);
+    }
+
+    public static <T> Function<FlagSource, JacksonFlag<T>> createUnbound(FlagId id, Class<T> jacksonClass, T defaultValue) {
+        return source -> new JacksonFlag<>(id, jacksonClass, defaultValue, source);
+    }
+
+    public JacksonFlag(String flagId, Class<T> jacksonClass, T defaultValue, FlagSource source) {
+        this(new FlagId(flagId), jacksonClass, defaultValue, source);
+    }
+
+    public JacksonFlag(FlagId id, Class<T> jacksonClass, T defaultValue, FlagSource source) {
+        this.id = id;
+        this.jacksonClass = jacksonClass;
+        this.defaultValue = defaultValue;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public T value() {
+        return source.getString(id).map(string -> uncheck(() -> mapper.readValue(string, jacksonClass))).orElse(defaultValue);
+    }
+
+    @Override
+    public String toString() {
+        return "JacksonFlag{" +
+                "id=" + id +
+                ", jacksonClass=" + jacksonClass +
+                ", defaultValue=" + defaultValue +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/LongFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/LongFlag.java
new file mode 100644
index 00000000000..d60dc7b5adc
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/LongFlag.java
@@ -0,0 +1,49 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import java.util.function.Function;
+
+/**
+ * @author hakonhall
+ */
+public class LongFlag implements Flag {
+    private final FlagId id;
+    private final long defaultValue;
+    private final FlagSource source;
+
+    public static Function<FlagSource, LongFlag> createUnbound(String flagId, int defaultValue) {
+        return createUnbound(new FlagId(flagId), defaultValue);
+    }
+
+    public static Function<FlagSource, LongFlag> createUnbound(FlagId id, int defaultValue) {
+        return source -> new LongFlag(id, defaultValue, source);
+    }
+
+    public LongFlag(String flagId, long defaultValue, FlagSource source) {
+        this(new FlagId(flagId), defaultValue, source);
+    }
+
+    public LongFlag(FlagId id, long defaultValue, FlagSource source) {
+        this.id = id;
+        this.defaultValue = defaultValue;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public long value() {
+        return source.getString(id).map(String::trim).map(Long::parseLong).orElse(defaultValue);
+    }
+
+    @Override
+    public String toString() {
+        return "LongFlag{" +
+                "id=" + id +
+                ", defaultValue=" + defaultValue +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/OptionalJacksonFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/OptionalJacksonFlag.java
new file mode 100644
index 00000000000..9b25a5d6786
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/OptionalJacksonFlag.java
@@ -0,0 +1,60 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import java.util.Optional;
+import java.util.function.Function;
+
+import static com.yahoo.yolean.Exceptions.uncheck;
+
+/**
+ * @author hakonhall
+ */
+public class OptionalJacksonFlag<T> implements Flag {
+    private final static ObjectMapper mapper = new ObjectMapper();
+
+    private final FlagId id;
+    private final Class<T> jacksonClass;
+    private final FlagSource source;
+
+    public static <T> Function<FlagSource, OptionalJacksonFlag<T>> createUnbound(String flagId, Class<T> jacksonClass) {
+        return createUnbound(new FlagId(flagId), jacksonClass);
+    }
+
+    public static <T> Function<FlagSource, OptionalJacksonFlag<T>> createUnbound(FlagId id, Class<T> jacksonClass) {
+        return source -> new OptionalJacksonFlag<>(id, jacksonClass, source);
+    }
+
+    public OptionalJacksonFlag(String flagId, Class<T> jacksonClass, FlagSource source) {
+        this(new FlagId(flagId), jacksonClass, source);
+    }
+
+    public OptionalJacksonFlag(FlagId id, Class<T> jacksonClass, FlagSource source) {
+        this.id = id;
+        this.jacksonClass = jacksonClass;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public Optional<T> value() {
+        return source.getString(id).map(string -> uncheck(() -> mapper.readValue(string, jacksonClass)));
+    }
+
+    public JacksonFlag<T> withDefault(T defaultValue) {
+        return new JacksonFlag<T>(id, jacksonClass, defaultValue, source);
+    }
+
+    @Override
+    public String toString() {
+        return "OptionalJacksonFlag{" +
+                "id=" + id +
+                ", jacksonClass=" + jacksonClass +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/OptionalStringFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/OptionalStringFlag.java
new file mode 100644
index 00000000000..3e25b08cf9d
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/OptionalStringFlag.java
@@ -0,0 +1,53 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import java.util.Optional;
+import java.util.function.Function;
+
+/**
+ * An OptionalStringFlag is a flag which is either not set (empty), or set (String present).
+ *
+ * @author hakonhall
+ */
+public class OptionalStringFlag implements Flag {
+    private final FlagId id;
+    private final FlagSource source;
+
+    public static Function<FlagSource, OptionalStringFlag> createUnbound(String flagId) {
+        return createUnbound(new FlagId(flagId));
+    }
+
+    public static Function<FlagSource, OptionalStringFlag> createUnbound(FlagId id) {
+        return source -> new OptionalStringFlag(id, source);
+    }
+
+    public OptionalStringFlag(String flagId, FlagSource source) {
+        this(new FlagId(flagId), source);
+    }
+
+    public OptionalStringFlag(FlagId id, FlagSource source) {
+        this.id = id;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public StringFlag bindDefault(String defaultValue) {
+        return new StringFlag(id, defaultValue, source);
+    }
+
+    public Optional<String> value() {
+        return source.getString(id);
+    }
+
+    @Override
+    public String toString() {
+        return "OptionalStringFlag{" +
+                "id=" + id +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/StringFlag.java b/flags/src/main/java/com/yahoo/vespa/flags/StringFlag.java
new file mode 100644
index 00000000000..8226e999238
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/StringFlag.java
@@ -0,0 +1,49 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import java.util.function.Function;
+
+/**
+ * @author hakonhall
+ */
+public class StringFlag implements Flag {
+    private final FlagId id;
+    private final String defaultValue;
+    private final FlagSource source;
+
+    public static Function<FlagSource, StringFlag> createUnbound(String flagId, String defaultValue) {
+        return createUnbound(new FlagId(flagId), defaultValue);
+    }
+
+    public static Function<FlagSource, StringFlag> createUnbound(FlagId id, String defaultValue) {
+        return source -> new StringFlag(id, defaultValue, source);
+    }
+
+    public StringFlag(String flagId, String defaultValue, FlagSource source) {
+        this(new FlagId(flagId), defaultValue, source);
+    }
+
+    public StringFlag(FlagId id, String defaultValue, FlagSource source) {
+        this.id = id;
+        this.defaultValue = defaultValue;
+        this.source = source;
+    }
+
+    @Override
+    public FlagId id() {
+        return id;
+    }
+
+    public String value() {
+        return source.getString(id).orElse(defaultValue);
+    }
+
+    @Override
+    public String toString() {
+        return "StringFlag{" +
+                "id=" + id +
+                ", defaultValue='" + defaultValue + '\'' +
+                ", source=" + source +
+                '}';
+    }
+}
diff --git a/flags/src/main/java/com/yahoo/vespa/flags/package-info.java b/flags/src/main/java/com/yahoo/vespa/flags/package-info.java
new file mode 100644
index 00000000000..42b9b057b29
--- /dev/null
+++ b/flags/src/main/java/com/yahoo/vespa/flags/package-info.java
@@ -0,0 +1,5 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+@ExportPackage
+package com.yahoo.vespa.flags;
+
+import com.yahoo.osgi.annotation.ExportPackage;
diff --git a/flags/src/test/java/com/yahoo/vespa/flags/FileFlagSourceTest.java b/flags/src/test/java/com/yahoo/vespa/flags/FileFlagSourceTest.java
new file mode 100644
index 00000000000..e422057f5fe
--- /dev/null
+++ b/flags/src/test/java/com/yahoo/vespa/flags/FileFlagSourceTest.java
@@ -0,0 +1,49 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import com.yahoo.vespa.test.file.TestFileSystem;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.nio.file.FileSystem;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class FileFlagSourceTest {
+    private final FileSystem fileSystem = TestFileSystem.create();
+    private final FileFlagSource source = new FileFlagSource(fileSystem);
+
+    @Test
+    public void absentThenSet() throws IOException {
+        FlagId id = new FlagId("foo");
+        FeatureFlag featureFlag = new FeatureFlag(id, source);
+        StringFlag stringFlag = new StringFlag(id, "default", source);
+        OptionalStringFlag optionalStringFlag = new OptionalStringFlag(id, source);
+        IntFlag intFlag = new IntFlag(id, -1, source);
+        LongFlag longFlag = new LongFlag(id, -2L, source);
+
+        assertFalse(source.hasFeature(id));
+        assertFalse(source.getString(id).isPresent());
+        assertFalse(featureFlag.isSet());
+        assertEquals("default", stringFlag.value());
+        assertFalse(optionalStringFlag.value().isPresent());
+        assertEquals(-1, intFlag.value());
+        assertEquals(-2L, longFlag.value());
+
+        Path featurePath = fileSystem.getPath(FileFlagSource.FLAGS_DIRECTORY).resolve(id.toString());
+        Files.createDirectories(featurePath.getParent());
+        Files.write(featurePath, "1\n".getBytes());
+
+        assertTrue(source.hasFeature(id));
+        assertTrue(source.getString(id).isPresent());
+        assertTrue(featureFlag.isSet());
+        assertEquals("1\n", stringFlag.value());
+        assertEquals("1\n", optionalStringFlag.value().get());
+        assertEquals(1, intFlag.value());
+        assertEquals(1L, longFlag.value());
+    }
+}
\ No newline at end of file
diff --git a/flags/src/test/java/com/yahoo/vespa/flags/JacksonFlagTest.java b/flags/src/test/java/com/yahoo/vespa/flags/JacksonFlagTest.java
new file mode 100644
index 00000000000..e4424d9886a
--- /dev/null
+++ b/flags/src/test/java/com/yahoo/vespa/flags/JacksonFlagTest.java
@@ -0,0 +1,72 @@
+// Copyright 2018 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.vespa.flags;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import org.junit.Test;
+
+import java.util.Objects;
+import java.util.Optional;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class JacksonFlagTest {
+    private final FlagId id = new FlagId("id");
+    private final ExampleJacksonClass defaultValue = new ExampleJacksonClass();
+    private final FlagSource source = mock(FlagSource.class);
+    private final JacksonFlag<ExampleJacksonClass> jacksonFlag = new JacksonFlag<>(id.toString(), ExampleJacksonClass.class, defaultValue, source);
+    private final OptionalJacksonFlag<ExampleJacksonClass> optionalJacksonFlag = new OptionalJacksonFlag<>(id, ExampleJacksonClass.class, source);
+
+    @Test
+    public void unsetThenSet() {
+        when(source.getString(id)).thenReturn(Optional.empty());
+        ExampleJacksonClass value = jacksonFlag.value();
+        assertEquals(1, value.integer);
+        assertEquals("2", value.string);
+        assertEquals("3", value.dummy);
+        assertFalse(optionalJacksonFlag.value().isPresent());
+
+        when(source.getString(id)).thenReturn(Optional.of("{\"integer\": 4, \"string\": \"foo\", \"stray\": 6}"));
+        value = jacksonFlag.value();
+        assertEquals(4, value.integer);
+        assertEquals("foo", value.string);
+        assertEquals("3", value.dummy);
+
+        assertTrue(optionalJacksonFlag.value().isPresent());
+        value = optionalJacksonFlag.value().get();
+        assertEquals(4, value.integer);
+        assertEquals("foo", value.string);
+        assertEquals("3", value.dummy);
+    }
+
+    @JsonIgnoreProperties(ignoreUnknown = true)
+    private static class ExampleJacksonClass {
+        @JsonProperty("integer")
+        public int integer = 1;
+
+        @JsonProperty("string")
+        public String string = "2";
+
+        @JsonProperty("dummy")
+        public String dummy = "3";
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+            ExampleJacksonClass that = (ExampleJacksonClass) o;
+            return integer == that.integer &&
+                    Objects.equals(string, that.string) &&
+                    Objects.equals(dummy, that.dummy);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(integer, string, dummy);
+        }
+    }
+}
\ No newline at end of file