14. sharding-jdbc源码之异常处理

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一般项目都会有自己的一套异常处理方式，sharding-jdbc也不以外，sharding-jdbc源码处理异常的方式主要有下面2种方式：
1. Preconditions
2. 自定义异常

1. Preconditions

google-guava的Preconditions用于条件检查，不符合预期的话则抛出异常，并可以重写异常信息。google-guava源码中Preconditions的注释如下：

Static convenience methods that help a method or constructor check whether it was invoked correctly (whether its preconditions have been met). These methods generally accept a boolean expression which is expected to be true (or in the case of checkNotNull, an object reference which is expected to be non-null). When false (or null) is passed instead, the Preconditions method throws an unchecked exception, which helps the calling method communicate to its caller that that caller has made a mistake. 

即帮助我们检查方法或者构造函数是否被正确调用，一般接收布尔表达式，期望布尔表达式的值为true；如果布尔表达式的值为false，就会抛出异常，让调用者知道错误的原因。

其部分static方法实现源码如下：
- 检查参数是否正确–expression就是判断方法的参数的表达式，errorMessage是自定义异常，不允许为空；

// Ensures the truth of an expression involving one or more parameters to the calling method.
public static void checkArgument(boolean expression, @Nullable Object errorMessage) {
    if (!expression) {
        throw new IllegalArgumentException(String.valueOf(errorMessage));
    }
}

• 检查状态是否正确–expression就是判断状态的参数的表达式，errorMessage是自定义异常，不允许为空；

// Ensures the truth of an expression involving the state of the calling instance, but not involving any parameters to the calling method.
public static void checkState(boolean expression, @Nullable Object errorMessage) {
    if (!expression) {
        throw new IllegalStateException(String.valueOf(errorMessage));
    }
}

• 检查不允许为空–reference就是待检查参数，errorMessage是自定义异常，不允许为空；

// Ensures that an object reference passed as a parameter to the calling method is not null.
public static <T> T checkNotNull(T reference, @Nullable Object errorMessage) {
    if (reference == null) {
        throw new NullPointerException(String.valueOf(errorMessage));
    }
    return reference;
}

• 检查下标是否越界–index就是待检查下标参数，size就是集合的size，errorMessage是自定义异常，不允许为空；

/**
 * Ensures that {@code index} specifies a valid <i>element</i> in an array, list or string of size
 * {@code size}. An element index may range from zero, inclusive, to {@code size}, exclusive.
 */
public static int checkElementIndex(
        int index, int size, @Nullable String desc) {
    // Carefully optimized for execution by hotspot (explanatory comment above)
    if (index < 0 || index >= size) {
        throw new IndexOutOfBoundsException(badElementIndex(index, size, desc));
    }
    return index;
}

接下来我们看一下sharding-jdbc源码里张亮大神是如何使用Preconditions的：
1. Preconditions.checkArgument()的使用
源码如下：

@SuppressWarnings("unchecked")
private <T extends ShardingStrategy> T buildShardingAlgorithmClassName(final List<String> shardingColumns, ... ...) {
    ... ...
    // 如果是SingleKeyShardingAlgorithm，那么sharding column只能有一个
    if (shardingAlgorithm instanceof SingleKeyShardingAlgorithm) {
        Preconditions.checkArgument(1 == shardingColumns.size(), "Sharding-JDBC: SingleKeyShardingAlgorithm must have only ONE sharding column");
        ... ...
    }
    ... ...
}

1. Preconditions.checkState()的使用
   源码如下：

private Collection<String> routeDataSources(final TableRule tableRule) {
    ... ...
    Collection<String> result = strategy.doStaticSharding(tableRule.getActualDatasourceNames(), shardingValues);
    // result是路由结果，即原生SQL路由后需要在哪些数据库中执行，很明显result肯定不可能为空；
    Preconditions.checkState(!result.isEmpty(), "no database route info");
    return result;
}

1. Preconditions.checkElementIndex()的使用
   源码如下（不是来自sharding-jdbc源码中，而是笔者写的）：

private static String getFromList(int index){
    // 如果从集合中取数据, 首先校验下标
    Preconditions.checkElementIndex(index, list.size(), "index is too big, list size is "+list.size()+". ");
    return list.get(index);
}

总结：很明显，借助google_guava的Preconditions能够让我们的代码更优雅，更简洁；

2. 自定义异常

sharding-jdbc自定义了异常处理类ShardingJdbcException：

public class ShardingJdbcException extends RuntimeException {

    // 异常类构造方法：异常信息errorMessage中有多个参数，例如：throw new ShardingJdbcException("Unsupported Date type:%s", convertType);
    public ShardingJdbcException(final String errorMessage, final Object... args) {
        super(String.format(errorMessage, args));
    }

    // 把catch的异常转成ShardingJdbcException类型的异常，并重写异常信息
    public ShardingJdbcException(final String message, final Exception cause) {
        super(message, cause);
    }

    // 把异常转成ShardingJdbcException类型的异常，不重写异常信息
    public ShardingJdbcException(final Exception cause) {
        super(cause);
    }
}

sharding-jdbc中抛出自定义日志场景

– 抛出自定义异常并重写有参数的异常信息

if (result.isEmpty()) {
    throw new ShardingJdbcException("Cannot find table rule and default data source with logic tables: '%s'", logicTables);
}

• 将IllegalAccessException或者InvocationTargetException类型的异常转化为ShardingJdbcException异常，并重写异常信息为”Invoke jdbc method exception”

try {
    method.invoke(target, arguments);
} catch (final IllegalAccessException | InvocationTargetException ex) {
    throw new ShardingJdbcException("Invoke jdbc method exception", ex);
}

• 把异常转成ShardingJdbcException类型的异常，不重写异常信息

public static void handleException(final Exception exception) {
    if (isExceptionThrown()) {
        throw new ShardingJdbcException(exception);
    }
    log.error("exception occur: ", exception);
}

总结

sharding-jdbc对异常的处理还是很有参考价值的，自定义异常类型封装业务异常，我们一般都会这么做；但是如果能借鉴sharding-jdbc的源码，再增加对Preconditions的使用，很明显能够让代码的逼格提升不少^^；