How to implement custom exceptions in Java?

Java中的异常

Java中默认的异常信息有哪些呢？Java程序中捕获异常之后会将异常进行输出，不知道细心的同学有没有注意到一点，输出的异常是什么东西呢？下面来看一个常见的ArithmeticException异常：

java.lang.ArithmeticException: / by zero
    at greenhouse.ExceptionTest.testException(ExceptionTest.java:16)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:157)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

再看看一个Java程序员耳熟能详的NullPointerException空指针异常:

java.lang.NullPointerException
    at greenhouse.ExceptionTest.testException(ExceptionTest.java:16)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:157)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

大家有没有发现一个特点，就是异常的输出中能够精确的输出异常出现的地点，精确到每一行代码，还有后面一大堆的执行过程类调用，也都打印出来了，这些信息从哪儿来呢？

这些信息是从栈中获取的，在打印异常日志的时候，会从JVM 栈中去获取这些调用信息。能够精确的定位异常出现的异常当然是好，但是我们有时候考虑到程序的性能，以及一些需求时，我们有时候并不需要完全的打印这些信息，并且去方法调用栈中获取相应的信息，是有性能消耗的，对于一些性能要求高的程序，我们完全可以在异常处理方面为程序性能做一个性能提升。

自定义Java异常类

所以如何避免输出这些堆栈信息呢？ 那么自定义异常就可以解决这个问题：

首先，自定义异常需要继承RuntimeException，然后，再通过是重写fillInStackTrace，toString 方法，例如下面我定义一个AppException异常:

package com.green.monitor.common.exception;

import java.text.MessageFormat;
/**
 * 自定义异常类
 */
public class AppException extends RuntimeException {

	private boolean isSuccess = false;
	private String key;
	private String info;

	public AppException(String key) {
		super(key);
		this.key = key;
		this.info = key;
	}

	public AppException(String key, String message) {
		super(MessageFormat.format("{0}[{1}]", key, message));
		this.key = key;
		this.info = message;
	}

	public AppException(String message, String key, String info) {
		super(message);
		this.key = key;
		this.info = info;
	}

	public boolean isSuccess() {
		return isSuccess;
	}

	public String getKey() {
		return key;
	}

	public void setKey(String key) {
		this.key = key;
	}

	public String getInfo() {
		return info;
	}

	public void setInfo(String info) {
		this.info = info;
	}

	@Override
	public Throwable fillInStackTrace() {
		return this;
	}

	@Override
	public String toString() {
		return MessageFormat.format("{0}[{1}]",this.key,this.info);
	}
}

Java异常源码

那么为什么要重写fillInStackTrace，和 toString 方法呢？ 我们首先来看源码是怎么一回事。

public class RuntimeException extends Exception {
    static final long serialVersionUID = -7034897190745766939L;

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

    /** Constructs a new runtime 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 RuntimeException(String message) {
        super(message);
    }

    /**
     * Constructs a new runtime exception with the specified detail message and
     * cause.  <p>Note that the detail message associated with
     * <code>cause</code> is <i>not</i> automatically incorporated in
     * this runtime exception&#39;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 <tt>null</tt> value is
     *         permitted, and indicates that the cause is nonexistent or
     *         unknown.)
     * @since  1.4
     */
    public RuntimeException(String message, Throwable cause) {
        super(message, cause);
    }

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

RuntimeException是继承Exception，但是它里面只是调用了父类的方法，本身是没有做什么其余的操作。那么继续看Exception里面是怎么回事。

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

    /**
     * Constructs a new exception with <code>null</code> 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</code> is <i>not</i> automatically incorporated in
     * this exception&#39;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 <tt>null</tt> 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 <tt>(cause==null ? null : cause.toString())</tt> (which
     * typically contains the class and detail message of <tt>cause</tt>).
     * 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 <tt>null</tt> value is
     *         permitted, and indicates that the cause is nonexistent or
     *         unknown.)
     * @since  1.4
     */
    public Exception(Throwable cause) {
        super(cause);
    }
}

从源码中可以看到，Exception里面也是直接调用了父类的方法，和RuntimeException一样，自己其实并没有做什么。那么直接来看Throwable里面是怎么一回事:

public class Throwable implements Serializable {
  public Throwable(String message) {
        fillInStackTrace();
        detailMessage = message;
    }
    
     /**
     * Fills in the execution stack trace. This method records within this
     * <code>Throwable</code> object information about the current state of
     * the stack frames for the current thread.
     *
     * @return  a reference to this <code>Throwable</code> instance.
     * @see     java.lang.Throwable#printStackTrace()
     */
    public synchronized native Throwable fillInStackTrace();
    
       /**
     * Provides programmatic access to the stack trace information printed by
     * {@link #printStackTrace()}.  Returns an array of stack trace elements,
     * each representing one stack frame.  The zeroth element of the array
     * (assuming the array&#39;s length is non-zero) represents the top of the
     * stack, which is the last method invocation in the sequence.  Typically,
     * this is the point at which this throwable was created and thrown.
     * The last element of the array (assuming the array&#39;s length is non-zero)
     * represents the bottom of the stack, which is the first method invocation
     * in the sequence.
     *
     * <p>Some virtual machines may, under some circumstances, omit one
     * or more stack frames from the stack trace.  In the extreme case,
     * a virtual machine that has no stack trace information concerning
     * this throwable is permitted to return a zero-length array from this
     * method.  Generally speaking, the array returned by this method will
     * contain one element for every frame that would be printed by
     * <tt>printStackTrace</tt>.
     *
     * @return an array of stack trace elements representing the stack trace
     *         pertaining to this throwable.
     * @since  1.4
     */
    public StackTraceElement[] getStackTrace() {
        return (StackTraceElement[]) getOurStackTrace().clone();
    }

    private synchronized StackTraceElement[] getOurStackTrace() {
        // Initialize stack trace if this is the first call to this method
        if (stackTrace == null) {
            int depth = getStackTraceDepth();
            stackTrace = new StackTraceElement[depth];
            for (int i=0; i < depth; i++)
                stackTrace[i] = getStackTraceElement(i);
        }
        return stackTrace;
    }
    
    /**
     * Returns the number of elements in the stack trace (or 0 if the stack
     * trace is unavailable).
     *
     * package-protection for use by SharedSecrets.
     */
    native int getStackTraceDepth();

    /**
     * Returns the specified element of the stack trace.
     *
     * package-protection for use by SharedSecrets.
     *
     * @param index index of the element to return.
     * @throws IndexOutOfBoundsException if <tt>index &lt; 0 ||
     *         index &gt;= getStackTraceDepth() </tt>
     */
    native StackTraceElement getStackTraceElement(int index);
    
    /**
     * Returns a short description of this throwable.
     * The result is the concatenation of:
     * <ul>
     * <li> the {@linkplain Class#getName() name} of the class of this object
     * <li> ": " (a colon and a space)
     * <li> the result of invoking this object&#39;s {@link #getLocalizedMessage}
     *      method
     * </ul>
     * If <tt>getLocalizedMessage</tt> returns <tt>null</tt>, then just
     * the class name is returned.
     *
     * @return a string representation of this throwable.
     */
    public String toString() {
        String s = getClass().getName();
        String message = getLocalizedMessage();
        return (message != null) ? (s + ": " + message) : s;
    }

从源码中可以看到，到Throwable就几乎到头了，在fillInStackTrace() 方法是一个native方法，这方法也就是会调用底层的C语言，返回一个Throwable对象，toString 方法，返回的是throwable的简短描述信息，并且在getStackTrace 方法和 getOurStackTrace 中调用的都是native方法getStackTraceElement，而这个方法是返回指定的栈元素信息，所以这个过程肯定是消耗性能的，那么我们自定义异常中的重写toString方法和fillInStackTrace方法就可以不从栈中去获取异常信息，直接输出，这样对系统和程序来说，相对就没有那么"重"，是一个优化性能的非常好的办法。

按照上面我们举例的自定义AppException异常，如果出现异常了，这个AppException异常输出是什么样的信息呢？请看下面吧:

@Test
    public void testException(){
        try {
            String str =null;
            System.out.println(str.charAt(0));
        }catch (Exception e){
            throw new AppException("000001","空指针异常");
        }
    }

执行上面单元测试，在异常异常的时候，系统将会打印我们自定义的异常信息:

000001[空指针异常]

Process finished with exit code -1

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