JDK源码分析 异常

Java

说明

对于JDK源码分析的文章，仅仅记录我认为重要的地方。源码的细节实在太多，不可能面面俱到地写清每个逻辑。所以我的JDK源码分析，着重在JDK的体系架构层面，具体源码可以参考：博客园：如果天空不死-博客。

例子

在开始研究异常的源码之前，首先看一下下面的示例：

@Test
public void testThrowable() {
    try {
        int a = 1 / 0;
    } catch (Exception e) {
        System.out.println(e.getMessage());
        System.out.println(e.getCause());
        e.printStackTrace();
    }
}

输出

/ by zero
null
java.lang.ArithmeticException: / by zero
    at test.jdk.testThrowable(jdk.java:383)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    ...

调试信息

异常的框架

Throwable

Throwable 是所有 errors 和 exceptions 的父类，只有继承了 Throwable 才能被 Java 语句中的异常捕获。一个 Throwable 对象包含线程创建到执行至异常处的所有栈。同时也包括一个额外的 message 信息，来描述异常。

Exception

Exception 也并没有什么代码，基本上就是继承了 Throwable 而已！重要的是 Exception 可以被捕获，而 Error 不需要也不能被捕获！

Error

Error 表示非常严重的错误，一个正常的程序不应该 catch 这个错误。这些错误是「非检查异常」，在编译期是无法发现的。

Throwable 源码分析

最重要的成员：

private StackTraceElement[] stackTrace = UNASSIGNED_STACK;

其中一个StackTraceElement代表一个栈帧，最顶端的StackTraceElement代表异常抛出地点，其余每个StackTraceElement都表示一个方法调用。

源码如下：

/**
 * An element in a stack trace, as returned by {@link
 * Throwable#getStackTrace()}.  Each element represents a single stack frame.
 * All stack frames except for the one at the top of the stack represent
 * a method invocation.  The frame at the top of the stack represents the
 * execution point at which the stack trace was generated.  Typically,
 * this is the point at which the throwable corresponding to the stack trace
 * was created.
 *
 * @since  1.4
 * @author Josh Bloch
 */
public final class StackTraceElement implements java.io.Serializable {
    // Normally initialized by VM (public constructor added in 1.5)
    private String declaringClass;
    private String methodName;
    private String fileName;
    private int    lineNumber;

UNASSIGNED_STACK 是一个空数组，用来表示未赋值的状态。

private static final StackTraceElement[] UNASSIGNED_STACK = new StackTraceElement[0];

主要看一个经常用到的用于打印堆栈信息的函数：printStackTrace。

private void printStackTrace(PrintStreamOrWriter s) {
    // Guard against malicious overrides of Throwable.equals by
    // using a Set with identity equality semantics.
    Set<Throwable> dejaVu =
        Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
    dejaVu.add(this);
    synchronized (s.lock()) {
        // Print our stack trace
        s.println(this);
        StackTraceElement[] trace = getOurStackTrace();
        for (StackTraceElement traceElement : trace)
            s.println("\tat " + traceElement);
        // Print suppressed exceptions, if any
        for (Throwable se : getSuppressed())
            se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
            // Print cause, if any
        Throwable ourCause = getCause();
        if (ourCause != null)
            ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
    }
}

Exception 源码分析

Exception 没有什么代码，只是继承了Throwable而已！

/**
 * The class {@code Exception} and its subclasses are a form of
 * {@code Throwable} that indicates conditions that a reasonable
 * application might want to catch.
 *
 * <p>The class {@code Exception} and any subclasses that are not also
 * subclasses of {@link RuntimeException} are <em>checked
 * exceptions</em>.  Checked exceptions need to be declared in a
 * method or constructor's {@code throws} clause if they can be thrown
 * by the execution of the method or constructor and propagate outside
 * the method or constructor boundary.
 *
 * @author  Frank Yellin
 * @see     java.lang.Error
 * @jls 11.2 Compile-Time Checking of Exceptions
 * @since   JDK1.0
 */
public class Exception extends Throwable {

Error 源码分析

Error 最重要的意义，在于 JVM 对它的约定。Error表示非常重要的错误，在编译期是无法检查出来的，程序不应该抛出Error。

/**
 * An {@code Error} is a subclass of {@code Throwable}
 * that indicates serious problems that a reasonable application
 * should not try to catch. Most such errors are abnormal conditions.
 * The {@code ThreadDeath} error, though a "normal" condition,
 * is also a subclass of {@code Error} because most applications
 * should not try to catch it.
 * <p>
 * A method is not required to declare in its {@code throws}
 * clause any subclasses of {@code Error} that might be thrown
 * during the execution of the method but not caught, since these
 * errors are abnormal conditions that should never occur.
 *
 * That is, {@code Error} and its subclasses are regarded as unchecked
 * exceptions for the purposes of compile-time checking of exceptions.
 *
 * @author  Frank Yellin
 * @see     java.lang.ThreadDeath
 * @jls 11.2 Compile-Time Checking of Exceptions
 * @since   JDK1.0
 */
public class Error extends Throwable {

总结

《Java编程思想》中提到：对于异常来说，最重要的是它的名字。异常的作用就是：用名字描述清楚所发生的错误，最好能够见名知意。用法也很简单：继承Exception即可！