This is because by default, FastAPI will inspect every item inside and make sure it is serializable as JSON, using the same [JSON Compatible Encoder](../tutorial/encoder.md){.internal-link target=_blank} explained in the tutorial. This is what allows you to return **arbitrary objects**, for example database models.

import com.google.errorprone.annotations.DoNotCall;
import com.google.errorprone.annotations.InlineMe;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.RandomAccess;

import com.google.common.reflect.Reflection;
import com.google.common.reflect.TypeToken;
import com.google.common.testing.NullPointerTester.Visibility;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Collection;

import com.google.j2objc.annotations.Weak;
import com.google.j2objc.annotations.WeakOuter;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractMap.SimpleEntry;
import java.util.AbstractSequentialList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;

import com.google.common.testing.ClassSanityTester.ParameterNotInstantiableException;
import com.google.common.testing.NullPointerTester.Visibility;
import com.google.errorprone.annotations.Keep;
import java.io.Serializable;
import java.lang.reflect.InvocationTargetException;
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;

 */
package org.codelibs.fess.crawler.rule;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;

import org.codelibs.fess.crawler.entity.ResponseData;

import com.google.errorprone.annotations.DoNotCall;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.WeakOuter;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.Objects;
import java.util.Set;

    suite.addTest(
        BiMapTestSuiteBuilder.using(new HashBiMapGenerator())
            .named("HashBiMap")
            .withFeatures(
                CollectionSize.ANY,
                CollectionFeature.SERIALIZABLE,
                CollectionFeature.SUPPORTS_ITERATOR_REMOVE,
                CollectionFeature.KNOWN_ORDER,
                MapFeature.ALLOWS_NULL_KEYS,
                MapFeature.ALLOWS_NULL_VALUES,

import com.google.common.testing.ClassSanityTester;
import com.google.common.testing.EqualsTester;
import com.google.common.testing.NullPointerTester;
import com.google.common.testing.SerializableTester;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.regex.Pattern;
import junit.framework.AssertionFailedError;

    return outer;
  }

  private static final class TestDeque<E> implements Deque<E> {
    private final Deque<E> delegate = new LinkedList<>();
    private final Object mutex = new Object[0]; // something Serializable

    @Override
    public boolean offer(E o) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.offer(o);
    }

    @Override
    public @Nullable E poll() {