diff --git a/spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/AbstractRoutingDataSourceTests.java b/spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/AbstractRoutingDataSourceTests.java
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+++ b/spring-jdbc/src/test/java/org/springframework/jdbc/datasource/lookup/AbstractRoutingDataSourceTests.java
@@ -0,0 +1,161 @@
+/*
+ * Copyright 2002-2021 the original author or authors.
+ *
+ * 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 org.springframework.jdbc.datasource.lookup;
+
+
+import java.util.HashMap;
+import java.util.Map;
+
+import javax.sql.DataSource;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+
+
+/**
+ * Tests for {@link AbstractRoutingDataSource}.
+ *
+ * @author Kazuki Shimizu
+ */
+class AbstractRoutingDataSourceTests {
+
+	@Test
+	void setTargetDataSources() {
+		final ThreadLocal<String> lookupKey = new ThreadLocal<>();
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return lookupKey.get();
+			}
+		};
+		DataSource ds1 = new StubDataSource();
+		DataSource ds2 = new StubDataSource();
+
+		MapDataSourceLookup dataSourceLookup = new MapDataSourceLookup();
+		dataSourceLookup.addDataSource("dataSource2", ds2);
+		routingDataSource.setDataSourceLookup(dataSourceLookup);
+
+		Map<Object, Object> targetDataSources = new HashMap<>();
+		targetDataSources.put("ds1", ds1);
+		targetDataSources.put("ds2", "dataSource2");
+		routingDataSource.setTargetDataSources(targetDataSources);
+
+		routingDataSource.afterPropertiesSet();
+		lookupKey.set("ds1");
+		assertThat(routingDataSource.determineTargetDataSource()).isSameAs(ds1);
+		lookupKey.set("ds2");
+		assertThat(routingDataSource.determineTargetDataSource()).isSameAs(ds2);
+	}
+
+	@Test
+	void targetDataSourcesIsNull() {
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return null;
+			}
+		};
+		assertThatIllegalArgumentException().isThrownBy(routingDataSource::afterPropertiesSet)
+				.withMessage("Property 'targetDataSources' is required");
+	}
+
+	@Test
+	void dataSourceIsUnSupportedType() {
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return null;
+			}
+		};
+		Map<Object, Object> targetDataSources = new HashMap<>();
+		targetDataSources.put("ds1", 1);
+		routingDataSource.setTargetDataSources(targetDataSources);
+		assertThatIllegalArgumentException().isThrownBy(routingDataSource::afterPropertiesSet)
+				.withMessage("Illegal data source value - only [javax.sql.DataSource] and String supported: 1");
+	}
+
+
+	@Test
+	void setDefaultTargetDataSource() {
+		final ThreadLocal<String> lookupKey = new ThreadLocal<>();
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return lookupKey.get();
+			}
+		};
+		DataSource ds = new StubDataSource();
+		routingDataSource.setTargetDataSources(new HashMap<>());
+		routingDataSource.setDefaultTargetDataSource(ds);
+		routingDataSource.afterPropertiesSet();
+		lookupKey.set("foo");
+		assertThat(routingDataSource.determineTargetDataSource()).isSameAs(ds);
+	}
+
+	@Test
+	void setDefaultTargetDataSourceFallbackIsFalse() {
+		final ThreadLocal<String> lookupKey = new ThreadLocal<>();
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return lookupKey.get();
+			}
+		};
+		DataSource ds = new StubDataSource();
+		routingDataSource.setTargetDataSources(new HashMap<>());
+		routingDataSource.setDefaultTargetDataSource(ds);
+		routingDataSource.setLenientFallback(false);
+		routingDataSource.afterPropertiesSet();
+		lookupKey.set("foo");
+		assertThatIllegalStateException().isThrownBy(routingDataSource::determineTargetDataSource)
+				.withMessage("Cannot determine target DataSource for lookup key [foo]");
+	}
+
+	@Test
+	void setDefaultTargetDataSourceLookupKeyIsNullWhenFallbackIsFalse() {
+		final ThreadLocal<String> lookupKey = new ThreadLocal<>();
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return lookupKey.get();
+			}
+		};
+		DataSource ds = new StubDataSource();
+		routingDataSource.setTargetDataSources(new HashMap<>());
+		routingDataSource.setDefaultTargetDataSource(ds);
+		routingDataSource.setLenientFallback(false);
+		routingDataSource.afterPropertiesSet();
+		lookupKey.set(null);
+		assertThat(routingDataSource.determineTargetDataSource()).isSameAs(ds);
+	}
+
+	@Test
+	public void notInitialized() {
+		AbstractRoutingDataSource routingDataSource = new AbstractRoutingDataSource() {
+			@Override
+			protected Object determineCurrentLookupKey() {
+				return null;
+			}
+		};
+		assertThatIllegalArgumentException().isThrownBy(routingDataSource::determineTargetDataSource)
+				.withMessage("DataSource router not initialized");
+	}
+
+}
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