refactor: transactionManager의 doInTransaction메서드가 반환값을 갖도록 수정

app/src/main/java/com/techcourse/service/TxUserService.java

@@ -25,6 +25,9 @@ public void insert(final User user) {
 
     @Override
     public void changePassword(final long id, final String newPassword, final String createBy) {
-        transactionManager.doInTransaction(() -> userService.changePassword(id, newPassword, createBy));
+        transactionManager.doInTransaction(() -> {
+            userService.changePassword(id, newPassword, createBy);
+            return null;
+        });
     }
 }

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

@@ -2,6 +2,8 @@
 
 import java.sql.Connection;
 import java.sql.SQLException;
+import java.util.function.Supplier;
+import javax.annotation.Nullable;
 import javax.sql.DataSource;
 import org.springframework.dao.DataAccessException;
 import org.springframework.jdbc.datasource.DataSourceUtils;
@@ -14,20 +16,23 @@ public TransactionManager(final DataSource dataSource) {
         this.dataSource = dataSource;
     }
 
-    public void doInTransaction(final Runnable runnable) throws DataAccessException {
+    @Nullable
+    public <T> T doInTransaction(final Supplier<T> supplier) throws DataAccessException {
+        T result = null;
         try (final var connection = DataSourceUtils.getConnection(dataSource)) {
             TransactionSynchronizationManager.bindResource(dataSource, connection);
             connection.setAutoCommit(false);
-            runnable.run();
+            result = supplier.get();
             connection.commit();
-        } catch (final SQLException e) {
+        } catch (final Exception e) {
             doRollback(e, TransactionSynchronizationManager.getResource(dataSource));
         } finally {
             TransactionSynchronizationManager.unbindResource(dataSource);
         }
+        return result;
     }
 
-    private void doRollback(final SQLException e, final Connection connection) {
+    private void doRollback(final Exception e, final Connection connection) {
         try {
             connection.rollback();
             throw new DataAccessException(e);

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

@@ -7,7 +7,7 @@
 
 public abstract class TransactionSynchronizationManager {
 
-    private static final ThreadLocal<Map<DataSource, Connection>> resources = new ThreadLocal<>();
+    private static final ThreadLocal<Map<DataSource, Connection>> resources = ThreadLocal.withInitial(HashMap::new);
 
     private TransactionSynchronizationManager() {
     }
@@ -25,13 +25,6 @@ static Connection unbindResource(final DataSource key) {
     }
 
     private static Map<DataSource, Connection> getThreadLocalResource() {
-        if (resources.get() == null) {
-            initSynchronization();
-        }
         return resources.get();
     }
-
-    private static void initSynchronization() {
-        resources.set(new HashMap<>());
-    }
 }