fix: 두 개 이상의 쓰레드에서 ThreadLocal에 접근할 때 NPE가 발생하는 문제 해결

jdbc/src/main/java/org/springframework/transaction/support/TransactionSynchronizationManager.java

@@ -1,21 +1,17 @@
 package org.springframework.transaction.support;
 
-import java.util.HashMap;
-import javax.sql.DataSource;
 import java.sql.Connection;
+import java.util.HashMap;
 import java.util.Map;
+import javax.sql.DataSource;
 
 public abstract class TransactionSynchronizationManager {
 
-    private static final ThreadLocal<Map<DataSource, Connection>> resources;
+    private static final ThreadLocal<Map<DataSource, Connection>> resources = ThreadLocal.withInitial(HashMap::new);
 
-    static {
-        resources = new ThreadLocal<>();
-        resources.set(new HashMap<>());
+    private TransactionSynchronizationManager() {
     }
 
-    private TransactionSynchronizationManager() {}
-
     public static Connection getResource(final DataSource key) {
         return resources.get().get(key);
     }

jdbc/src/test/java/org/springframework/transaction/support/TransactionSynchronizationManagerTest.java

@@ -1,7 +1,6 @@
 package org.springframework.transaction.support;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 import java.sql.Connection;
 import java.sql.SQLException;
@@ -11,12 +10,19 @@
 
 class TransactionSynchronizationManagerTest {
 
+    @Test
+    void 저장된_데이터소스가_없는_경우_null이_반환된다() {
+        Connection connection = TransactionSynchronizationManager.getResource(TestDataSourceConfig.getInstance());
+        assertThat(connection).isNull();
+    }
+
     @Test
     void 쓰레드별로_동일한_데이터소스를_가져도_서로_다른_리소스를_가진다() throws InterruptedException {
         DataSource dataSource = TestDataSourceConfig.getInstance();
         new Thread(() -> {
             try {
-                TransactionSynchronizationManager.bindResource(dataSource, dataSource.getConnection()); // 한 쓰레드에서 connection 바인딩
+                TransactionSynchronizationManager.bindResource(dataSource,
+                        dataSource.getConnection()); // 한 쓰레드에서 connection 바인딩
             } catch (SQLException e) {
                 throw new RuntimeException(e);
             }
@@ -25,7 +31,6 @@ class TransactionSynchronizationManagerTest {
         }).start();
 
         Thread.sleep(500);
-        assertThatThrownBy(() -> TransactionSynchronizationManager.getResource(dataSource)) // 동일한 키 값으로 조회하더라도 쓰레드가 달라서 NPE 발생
-                .isInstanceOf(NullPointerException.class);
+        assertThat(TransactionSynchronizationManager.getResource(dataSource)).isNull(); // 동일한 키 값으로 조회하더라도 쓰레드가 달라서 null 발생
     }
 }