我正在尝试向 Android 设备发送推送通知，但在以下情况下出现错误。
String msg = "push message";
String to1 = "123456";
String from1 = "098776655";
这工作正常：
Message message = new Message.Builder()
.collapseKey("1")
.addData("message", msg)
.addData("to1", to1)
.addData("to2", to1)
.build();
但是使用以下内容时出现错误：
Message message = new Message.Builder()
.collapseKey("1")
.addData("message", msg)
.addData("to", to1)
.addData("from", from1)
.build();
我什至也使用了其他变量来代替from1，但仍然出现错误：
java.io.IOException: Server returned HTTP response code: 400 for URL: https://android.googleapis.com/gcm/send
堆栈跟踪：
java.io.IOException: Server returned HTTP response code: 400 for URL: https://android.googleapis.com/gcm/send
[run] at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
[run] at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
[run] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
[run] at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
[run] at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1298)
[run] at java.security.AccessController.doPrivileged(Native Method)
[run] at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1292)
[run] at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:948)
[run] at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:234)
[run] at com.google.android.gcm.server.Sender.sendNoRetry(Sender.java:363)
[run] at com.google.android.gcm.server.Sender.send(Sender.java:261)
[run] at example.HelloServlet.sendToSingleDevice(HelloServlet.java:173)
[run] at example.HelloServlet.doGet(HelloServlet.java:36)
[run] at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
[run] at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
[run] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[run] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
[run] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
[run] at java.lang.reflect.Method.invoke(Method.java:597)
[run] at org.apache.catalina.security.SecurityUtil$1.run(SecurityUtil.java:270)
[run] at org.apache.catalina.security.SecurityUtil$1.run(SecurityUtil.java:269)
[run] at java.security.AccessController.doPrivileged(Native Method)
[run] at javax.security.auth.Subject.doAsPrivileged(Subject.java:517)
[run] at org.apache.catalina.security.SecurityUtil.execute(SecurityUtil.java:302)
[run] at org.apache.catalina.security.SecurityUtil.doAsPrivilege(SecurityUtil.java:163)
[run] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:283)
[run] at org.apache.catalina.core.ApplicationFilterChain.access$000(ApplicationFilterChain.java:56)
[run] at org.apache.catalina.core.ApplicationFilterChain$1.run(ApplicationFilterChain.java:189)
[run] at java.security.AccessController.doPrivileged(Native Method)
[run] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:185)
[run] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
[run] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
[run] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
[run] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
[run] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
[run] at net.stax.appserver.webapp.RequestMonitorValve.invoke(RequestMonitorValve.java:35)
[run] at net.stax.appserver.webapp.RequestSetupValve.invoke(RequestSetupValve.java:31)
[run] at org.apache.catalina.valves.RemoteIpValve.invoke(RemoteIpValve.java:647)
[run] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
[run] at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
[run] at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:602)
[run] at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
[run] at java.lang.Thread.run(Thread.java:619)
[run] Caused by: java.io.IOException: Server returned HTTP response code: 400 for URL: https://android.googleapis.com/gcm/send
[run] at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1245)
[run] at java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:373)
[run] at sun.net.www.protocol.https.HttpsURLConnectionImpl.getResponseCode(HttpsURLConnectionImpl.java:318)
[run] at com.google.android.gcm.server.Sender.sendNoRetry(Sender.java:362)
[run] ... 33 more
[run] Exception Message : Server returned HTTP response code: 400 for URL: https://android.googleapis.com/gcm/send