The realization principle of Springboot transaction
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Spring's implementation of transactions is divided into programmatic and declarative. The declarative is the most common. The implementation of declarative transactions is divided into XML configuration files and @Transactional annotations. SpringBoot cancels the writing of XML configuration files. , So the transaction in SpringBoot is mainly realized by using @Transactional annotation. The principle is analyzed below from the source code point of view
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It is known that the principle of @Transactional transaction implementation is based on AOP. In the previous blog, the realization principle of AOP was explained in detail: dynamic proxy + interception chain. From this, you can roughly infer the implementation logic of @Transactional: Spring has a target @Transactional's enhancer (interceptor) Interceptor, in the last step of bean instance initialization, the interceptor chain with the interceptor will be called to enhance the @Transactional annotation method, and the proxy class will be generated
@Transactional annotation
/**
* Alias for {@link #transactionManager}.
* @see #transactionManager
*/
@AliasFor("transactionManager")
String value() default "";
/**
* A <em>qualifier</em> value for the specified transaction.
* <p>May be used to determine the target transaction manager,
* matching the qualifier value (or the bean name) of a specific
* {@link org.springframework.transaction.PlatformTransactionManager}
* bean definition.
* @since 4.2
* @see #value
*/
@AliasFor("value")
String transactionManager() default "";
/**
* The transaction propagation type.
* <p>Defaults to {@link Propagation#REQUIRED}.
* @see org.springframework.transaction.interceptor.TransactionAttribute#getPropagationBehavior()
*/
Propagation propagation() default Propagation.REQUIRED;
/**
* The transaction isolation level.
* <p>Defaults to {@link Isolation#DEFAULT}.
* @see org.springframework.transaction.interceptor.TransactionAttribute#getIsolationLevel()
*/
Isolation isolation() default Isolation.DEFAULT;
/**
* The timeout for this transaction.
* <p>Defaults to the default timeout of the underlying transaction system.
* @see org.springframework.transaction.interceptor.TransactionAttribute#getTimeout()
*/
int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;
/**
* {@code true} if the transaction is read-only.
* <p>Defaults to {@code false}.
* <p>This just serves as a hint for the actual transaction subsystem;
* it will <i>not necessarily</i> cause failure of write access attempts.
* A transaction manager which cannot interpret the read-only hint will
* <i>not</i> throw an exception when asked for a read-only transaction
* but rather silently ignore the hint.
* @see org.springframework.transaction.interceptor.TransactionAttribute#isReadOnly()
*/
boolean readOnly() default false;
/**
* Defines zero (0) or more exception {@link Class classes}, which must be
* subclasses of {@link Throwable}, indicating which exception types must cause
* a transaction rollback.
* <p>By default, a transaction will be rolling back on {@link RuntimeException}
* and {@link Error} but not on checked exceptions (business exceptions). See
* {@link org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)}
* for a detailed explanation.
* <p>This is the preferred way to construct a rollback rule (in contrast to
* {@link #rollbackForClassName}), matching the exception class and its subclasses.
* <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(Class clazz)}.
* @see #rollbackForClassName
* @see org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)
*/
Class<? extends Throwable>[] rollbackFor() default {
};
/**
* Defines zero (0) or more exception names (for exceptions which must be a
* subclass of {@link Throwable}), indicating which exception types must cause
* a transaction rollback.
* <p>This can be a substring of a fully qualified class name, with no wildcard
* support at present. For example, a value of {@code "ServletException"} would
* match {@code javax.servlet.ServletException} and its subclasses.
* <p><b>NB:</b> Consider carefully how specific the pattern is and whether
* to include package information (which isn't mandatory). For example,
* {@code "Exception"} will match nearly anything and will probably hide other
* rules. {@code "java.lang.Exception"} would be correct if {@code "Exception"}
* were meant to define a rule for all checked exceptions. With more unusual
* {@link Exception} names such as {@code "BaseBusinessException"} there is no
* need to use a FQN.
* <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(String exceptionName)}.
* @see #rollbackFor
* @see org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)
*/
String[] rollbackForClassName() default {
};
/**
* Defines zero (0) or more exception {@link Class Classes}, which must be
* subclasses of {@link Throwable}, indicating which exception types must
* <b>not</b> cause a transaction rollback.
* <p>This is the preferred way to construct a rollback rule (in contrast
* to {@link #noRollbackForClassName}), matching the exception class and
* its subclasses.
* <p>Similar to {@link org.springframework.transaction.interceptor.NoRollbackRuleAttribute#NoRollbackRuleAttribute(Class clazz)}.
* @see #noRollbackForClassName
* @see org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)
*/
Class<? extends Throwable>[] noRollbackFor() default {
};
/**
* Defines zero (0) or more exception names (for exceptions which must be a
* subclass of {@link Throwable}) indicating which exception types must <b>not</b>
* cause a transaction rollback.
* <p>See the description of {@link #rollbackForClassName} for further
* information on how the specified names are treated.
* <p>Similar to {@link org.springframework.transaction.interceptor.NoRollbackRuleAttribute#NoRollbackRuleAttribute(String exceptionName)}.
* @see #noRollbackFor
* @see org.springframework.transaction.interceptor.DefaultTransactionAttribute#rollbackOn(Throwable)
*/
String[] noRollbackForClassName() default {
};
Transaction isolation level
/**
* Use the default isolation level of the underlying datastore.
* All other levels correspond to the JDBC isolation levels.
* @see java.sql.Connection
*/
DEFAULT(TransactionDefinition.ISOLATION_DEFAULT),
/**
* A constant indicating that dirty reads, non-repeatable reads and phantom reads
* can occur. This level allows a row changed by one transaction to be read by
* another transaction before any changes in that row have been committed
* (a "dirty read"). If any of the changes are rolled back, the second
* transaction will have retrieved an invalid row.
* @see java.sql.Connection#TRANSACTION_READ_UNCOMMITTED
*/
READ_UNCOMMITTED(TransactionDefinition.ISOLATION_READ_UNCOMMITTED),
/**
* A constant indicating that dirty reads are prevented; non-repeatable reads
* and phantom reads can occur. This level only prohibits a transaction
* from reading a row with uncommitted changes in it.
* @see java.sql.Connection#TRANSACTION_READ_COMMITTED
*/
READ_COMMITTED(TransactionDefinition.ISOLATION_READ_COMMITTED),
/**
* A constant indicating that dirty reads and non-repeatable reads are
* prevented; phantom reads can occur. This level prohibits a transaction
* from reading a row with uncommitted changes in it, and it also prohibits
* the situation where one transaction reads a row, a second transaction
* alters the row, and the first transaction rereads the row, getting
* different values the second time (a "non-repeatable read").
* @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
*/
REPEATABLE_READ(TransactionDefinition.ISOLATION_REPEATABLE_READ),
/**
* A constant indicating that dirty reads, non-repeatable reads and phantom
* reads are prevented. This level includes the prohibitions in
* {@code ISOLATION_REPEATABLE_READ} and further prohibits the situation
* where one transaction reads all rows that satisfy a {@code WHERE}
* condition, a second transaction inserts a row that satisfies that
* {@code WHERE} condition, and the first transaction rereads for the
* same condition, retrieving the additional "phantom" row in the second read.
* @see java.sql.Connection#TRANSACTION_SERIALIZABLE
*/
SERIALIZABLE(TransactionDefinition.ISOLATION_SERIALIZABLE);
TransactionInterceptor interceptor
Regarding how the interceptor chain is formed and executed, I won't go into details here, and I will mainly focus on interceptors for @Transactional annotations.
@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
// Work out the target class: may be {@code null}.
// The TransactionAttributeSource should be passed the target class
// as well as the method, which may be from an interface.
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
// Adapt to TransactionAspectSupport's invokeWithinTransaction...
return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
@Override
public Object proceedWithInvocation() throws Throwable {
return invocation.proceed();
}
});
}
- Follow the invokeWithinTransaction function to save unnecessary code. It can be clearly seen here that the interceptor implements the transaction in the same way as we manually implement the transaction at the DAO layer. It is to start the transaction first, then execute it, and roll back if it is abnormal. , Otherwise commit the transaction, but here replace the execution logic with a recursive execution interception chain
/**
* General delegate for around-advice-based subclasses, delegating to several other template
* methods on this class. Able to handle {@link CallbackPreferringPlatformTransactionManager}
* as well as regular {@link PlatformTransactionManager} implementations.
* @param method the Method being invoked
* @param targetClass the target class that we're invoking the method on
* @param invocation the callback to use for proceeding with the target invocation
* @return the return value of the method, if any
* @throws Throwable propagated from the target invocation
*/
protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
throws Throwable {
// If the transaction attribute is null, the method is non-transactional.
final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
final PlatformTransactionManager tm = determineTransactionManager(txAttr);
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
// Standard transaction demarcation with getTransaction and commit/rollback calls.
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal = null;
try {
// This is an around advice: Invoke the next interceptor in the chain.
// This will normally result in a target object being invoked.
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally {
cleanupTransactionInfo(txInfo);
}
commitTransactionAfterReturning(txInfo);
return retVal;
}
else {
final ThrowableHolder throwableHolder = new ThrowableHolder();
// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
try {
Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr,
new TransactionCallback<Object>() {
@Override
public Object doInTransaction(TransactionStatus status) {
TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
try {
return invocation.proceedWithInvocation();
}
catch (Throwable ex) {
if (txAttr.rollbackOn(ex)) {
// A RuntimeException: will lead to a rollback.
if (ex instanceof RuntimeException) {
throw (RuntimeException) ex;
}
else {
throw new ThrowableHolderException(ex);
}
}
else {
// A normal return value: will lead to a commit.
throwableHolder.throwable = ex;
return null;
}
}
finally {
cleanupTransactionInfo(txInfo);
}
}
});
// Check result state: It might indicate a Throwable to rethrow.
if (throwableHolder.throwable != null) {
throw throwableHolder.throwable;
}
return result;
}
catch (ThrowableHolderException ex) {
throw ex.getCause();
}
catch (TransactionSystemException ex2) {
if (throwableHolder.throwable != null) {
logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
ex2.initApplicationException(throwableHolder.throwable);
}
throw ex2;
}
catch (Throwable ex2) {
if (throwableHolder.throwable != null) {
logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
}
throw ex2;
}
}
}