在学习Java的过程中，想必大家都一定学习过异常这个篇章，异常的基本特性和使用这里就不再多讲了。想必大家都能够理解看懂，并正确使用。
但是，光学会基本异常处理和使用不够的，在工作中常会有自定义业务异常的场景，根据不同的业务异常做对应异常处理，出现异常并不可怕，有时候是需要使用异常来驱动业务的处理，例如: 在使用唯一约束的数据库的时候，如果插入一条重复的数据，那么可以通过捕获唯一约束异常DuplicateKeyException，如果出现CommunicationsException是不是又要去处理呢？如果两种情况都使用同样业务逻辑来处理，是不是同样捕获呢？这个时候，其实如果在DAO层统一捕获Exception，然后向上抛出自定义异常，在调用成层根据对应的业务异常再进行处理，而且自定义异常能做很多轻量化处理(请看下文解释)，是不是方便很多呢？所以这里自定义业务异常：既是对业务不同异常场景下的区分，又是通过异常来驱动业务流程的处理，以自定义异常好处很多。
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'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'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'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'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 < 0 ||
* index >= 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'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
所以特别简洁，优化了系统程序性能，让程序不这么“重”，所以对于性能要求特别要求的系统，赶紧自定义业务异常试一试吧！
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