记1个低级错误：java.io.IOException: Server returned HTTP response code: 500 for URL: http://localhost:8769/accr/print

这样一个简单方法，

    public void as() throws Exception{
        URL url = new URL("http://localhost:8769/accr/print");
        HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
        urlConnection.setDoInput(true);
        urlConnection.connect(); 
        //获取状态码
        int code = urlConnection.getResponseCode();
        System.out.println(code);
        InputStream in = urlConnection.getInputStream();
        InputStream raw;
        if (code == 200) {
            raw = new BufferedInputStream(in);
        } else {
            raw = urlConnection.getErrorStream();// HttpURLConnection 才有getErrorStream方法
        }
        //放入缓存流
        //最后使用Reader接收
        Reader r = new InputStreamReader(raw);
        //打印输出
        int c;
        while((c = r.read())>0){
            System.out.print((char)c);
        }
        System.out.println();
    }

竟然报错：

java.io.IOException: Server returned HTTP response code: 500 for URL: http://localhost:8769/accr/print
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1950)
    at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1945)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1944)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1514)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1498)
    at com.sa.bytebuddy.Test.as(Test.java:183)
    at com.sa.bytebuddy.Test.testSmoothBurstyRed(Test.java:94)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$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)
Caused by: java.io.IOException: Server returned HTTP response code: 500 for URL: http://localhost:8769/accr/print
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1900)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1498)
    at java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:480)
    at com.sa.bytebuddy.Test.as(Test.java:181)
    ... 23 more

其实原因很简单， URLConnection connect 之后返回 500 的话，是不能执行 getInputStream 方法的。

需要使用getErrorStream方法，但是 URLConnection没有， HttpURLConnection 才有getErrorStream方法。 强制转换一下即可。

HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

然后，需要判断 statusCode 是不是200， 否则应该使用

        //获取状态码
        int code = urlConnection.getResponseCode();
        System.out.println(code);
        InputStream raw;
        if (code == 200) {
            InputStream in = urlConnection.getInputStream();
            raw = new BufferedInputStream(in);
        } else {
            raw = urlConnection.getErrorStream();// HttpURLConnection 才有getErrorStream方法
        }