Java Exceptions & Errors

1. Java 异常类的层次结构

图片摘自 Simple Snippets

从图中可以看出， Exceptions 类和 Errors 类都继承于 Throwable 类。

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2. Java Exception

直接上源码（摘自 JDK 11.0.10 ）

public class Exception extends Throwable {
    static final long serialVersionUID = -3387516993124229948L;

    /**
     * Constructs a new exception with {@code null} as its detail message.
     * The cause is not initialized, and may subsequently be initialized by a
     * call to {@link #initCause}.
     */
    public Exception() {
        super();
    }

    /**
     * Constructs a new exception with the specified detail message.  The
     * cause is not initialized, and may subsequently be initialized by
     * a call to {@link #initCause}.
     *
     * @param   message   the detail message. The detail message is saved for
     *          later retrieval by the {@link #getMessage()} method.
     */
    public Exception(String message) {
        super(message);
    }

    /**
     * Constructs a new exception with the specified detail message and
     * cause.  <p>Note that the detail message associated with
     * {@code cause} is <i>not</i> automatically incorporated in
     * this exception's detail message.
     *
     * @param  message the detail message (which is saved for later retrieval
     *         by the {@link #getMessage()} method).
     * @param  cause the cause (which is saved for later retrieval by the
     *         {@link #getCause()} method).  (A {@code null} value is
     *         permitted, and indicates that the cause is nonexistent or
     *         unknown.)
     * @since  1.4
     */
    public Exception(String message, Throwable cause) {
        super(message, cause);
    }

    /**
     * Constructs a new exception with the specified cause and a detail
     * message of {@code (cause==null ? null : cause.toString())} (which
     * typically contains the class and detail message of {@code cause}).
     * This constructor is useful for exceptions that are little more than
     * wrappers for other throwables (for example, {@link
     * java.security.PrivilegedActionException}).
     *
     * @param  cause the cause (which is saved for later retrieval by the
     *         {@link #getCause()} method).  (A {@code null} value is
     *         permitted, and indicates that the cause is nonexistent or
     *         unknown.)
     * @since  1.4
     */
    public Exception(Throwable cause) {
        super(cause);
    }

    /**
     * Constructs a new exception with the specified detail message,
     * cause, suppression enabled or disabled, and writable stack
     * trace enabled or disabled.
     *
     * @param  message the detail message.
     * @param cause the cause.  (A {@code null} value is permitted,
     * and indicates that the cause is nonexistent or unknown.)
     * @param enableSuppression whether or not suppression is enabled
     *                          or disabled
     * @param writableStackTrace whether or not the stack trace should
     *                           be writable
     * @since 1.7
     */
    protected Exception(String message, Throwable cause,
                        boolean enableSuppression,
                        boolean writableStackTrace) {
        super(message, cause, enableSuppression, writableStackTrace);
    }
}

​ 在 Exception(异常) 类中，可以分为 Check Exceptions 和 Uncheck Exceptions(Runtime Exception) 。异常可由程序自行处理。其中检查异常是在程序编写过程中必须处理的（不处理无法通过编译），不受检查异常在编译过程中无法发现。

2.1 Check Exceptions

​ 这类异常必须由 try / catch 进行捕获，如果不进行捕获，则不能通过编译。常见的 Check Exceptions有： IOException / ClassNotFoundExcepiton / SQLException / …

2.2 Uncheck Exceptions

​ RuntimException及其子类都属于 Uncheck Exception。常见的 Uncheck Exception 有：NullPointerException、NumberFormatException(字符串转换为数字)、ArrayIndexOutOfBoundsException(数组越界)、ClassCastException(类型转换错误)、ArithmeticException(算术错误)

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3. Errors

​ Error：Error属于程序无法处理的错误，这类错误无法用 catch来进行捕获。比如 JVM 虚拟机错误 / JVM 虚拟机内存不足 / 类定义错误 / … 当发生这种异常时， JVM 一般会线程终止。

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4. try - catch -finall

• try: 用于执行可能出现异常的代码块，后面会跟着 [0-n]个catch块
• catch: 用于处理 try块中捕获到的异常
• finally: 用于之后的工作，不论是否抛出异常，finally块内的代码都会被执行

特殊情况（finally 块不执行）： – 参考 Java Guide Blog

• 在 try 或 finally 块中用了 System.exit(int)退出程序。如果 System.exit(int) 在异常语句之后，finally 还是会被执行

• 程序所在的线程死亡

• CPU 关闭

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5. try - with - resources

​ 相较于 try - catch 语句 try - with - resources主要用于关闭资源（即实现 java.lang.AutoCloseable或者 java.io.Closeable 的对象）
​ 如果有多个资源需要关闭时，多个资源之间需要使用 分号 分隔。

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