WSO2 ESB: Unexpected Character Error when submitting JSON to Proxy Service

Question

I'm attempting to retrieve data from Facebook using the WSO2 Facebook Connector in WSO2 ESB 4.9.0. I've provided the proxy configuration below.

<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
       name="facebook_getFeed"
       transports="https,http"
       statistics="disable"
       trace="disable"
       startOnLoad="true">
   <target>
      <inSequence onError="fault">
         <property name="messageType" value="application/json" scope="axis2"/>
         <log level="full"/>
         <property name="apiUrl" expression="json-eval($.apiUrl)"/>
         <property name="apiVersion" expression="json-eval($.apiVersion)"/>
         <property name="accessToken" expression="json-eval($.accessToken)"/>
         <property name="userId" expression="json-eval($.userId)"/>
         <property name="numOfFeeds" expression="json-eval($.numOfFeeds)"/>
         <property name="fields" expression="json-eval($.fields)"/>
         <facebook.init>
            <apiUrl>{$ctx:apiUrl}</apiUrl>
            <accessToken>{$ctx:accessToken}</accessToken>
            <apiVersion>{$ctx:apiVersion}</apiVersion>
         </facebook.init>
         <facebook.getFeeds>
            <userId>{$ctx:userId}</userId>
            <numOfFeeds>{$ctx:numOfFeeds}</numOfFeeds>
            <fields>{$ctx:fields}</fields>
         </facebook.getFeeds>
         <respond/>
      </inSequence>
      <outSequence>
         <property name="messageType" value="application/json" scope="axis2"/>
         <send/>
      </outSequence>
   </target>
   <description/>
</proxy>

The issue I'm having is that whenever I make a POST request to the service it times out. Checking the ESB error logs it appears that there was an "Error While building Passthrough stream".

[2015-12-15 15:16:54,284] ERROR - RelayUtils Error while building Passthrough stream
org.apache.axiom.om.OMException: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:296)
        at org.apache.axiom.om.impl.llom.OMDocumentImpl.getOMDocumentElement(OMDocumentImpl.java:109)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:570)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:566)
        at org.apache.synapse.transport.passthru.util.DeferredMessageBuilder.getDocument(DeferredMessageBuilder.java:149)
        at org.apache.synapse.transport.passthru.util.RelayUtils.builldMessage(RelayUtils.java:133)
        at org.apache.synapse.transport.passthru.util.RelayUtils.buildMessage(RelayUtils.java:98)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:72)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:48)
        at org.apache.synapse.mediators.base.SequenceMediator.mediate(SequenceMediator.java:149)
        at org.apache.synapse.core.axis2.ProxyServiceMessageReceiver.receive(ProxyServiceMessageReceiver.java:185)
        at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:180)
        at org.apache.synapse.transport.passthru.ServerWorker.processNonEntityEnclosingRESTHandler(ServerWorker.java:317)
        at org.apache.synapse.transport.passthru.ServerWorker.processEntityEnclosingRequest(ServerWorker.java:363)
        at org.apache.synapse.transport.passthru.ServerWorker.run(ServerWorker.java:142)
        at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at com.ctc.wstx.sr.StreamScanner.throwUnexpectedChar(StreamScanner.java:639)
        at com.ctc.wstx.sr.BasicStreamReader.nextFromProlog(BasicStreamReader.java:2052)
        at com.ctc.wstx.sr.BasicStreamReader.next(BasicStreamReader.java:1134)
        at org.apache.axiom.util.stax.wrapper.XMLStreamReaderWrapper.next(XMLStreamReaderWrapper.java:225)
        at org.apache.axiom.util.stax.dialect.DisallowDoctypeDeclStreamReaderWrapper.next(DisallowDoctypeDeclStreamReaderWrapper.java:34)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.parserNext(StAXOMBuilder.java:681)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:214)
        ... 18 more
[2015-12-15 15:16:54,377] ERROR - SequenceMediator Error while building message
org.apache.axis2.AxisFault: Error while building Passthrough stream
        at org.apache.synapse.transport.passthru.util.RelayUtils.handleException(RelayUtils.java:283)
        at org.apache.synapse.transport.passthru.util.RelayUtils.builldMessage(RelayUtils.java:142)
        at org.apache.synapse.transport.passthru.util.RelayUtils.buildMessage(RelayUtils.java:98)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:72)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:48)
        at org.apache.synapse.mediators.base.SequenceMediator.mediate(SequenceMediator.java:149)
        at org.apache.synapse.core.axis2.ProxyServiceMessageReceiver.receive(ProxyServiceMessageReceiver.java:185)
        at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:180)
        at org.apache.synapse.transport.passthru.ServerWorker.processNonEntityEnclosingRESTHandler(ServerWorker.java:317)
        at org.apache.synapse.transport.passthru.ServerWorker.processEntityEnclosingRequest(ServerWorker.java:363)
        at org.apache.synapse.transport.passthru.ServerWorker.run(ServerWorker.java:142)
        at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.axiom.om.OMException: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:296)
        at org.apache.axiom.om.impl.llom.OMDocumentImpl.getOMDocumentElement(OMDocumentImpl.java:109)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:570)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:566)
        at org.apache.synapse.transport.passthru.util.DeferredMessageBuilder.getDocument(DeferredMessageBuilder.java:149)
        at org.apache.synapse.transport.passthru.util.RelayUtils.builldMessage(RelayUtils.java:133)
        ... 13 more

Referring to the proxy configuration above, I've got a log mediator which actually successfully prints the JSON payload, so that would lead me to think it has something to do with the messager formatter?

My message formatter/builder configuration within the Axis2.xml file is as follows:

<!--JSON Message Formatters-->
<messageFormatter contentType="application/json" class="org.apache.synapse.commons.json.JsonStreamFormatter"/>

<!--JSON Message Builders-->
<messageBuilder contentType="application/json" class="org.apache.synapse.commons.json.JsonStreamBuilder"/>

Everything else in this file is as per the default configuration.

What could be going wrong here?

Sample Request

Using Soap UI (Obscured IP and access token in payload):

POST http://[SERVER_IP]:8280/services/facebook_getFeed <br>
HTTP/1.1 <br>
Accept-Encoding: gzip,deflate <br>
Content-Type: application/json <br>
Content-Length: 207 <br>
Host: [SERVER_IP]:8280 <br>
Connection: Keep-Alive <br>
User-Agent: Apache-HttpClient/4.1.1 (java 1.5) <br>

Payload Data:

{
"apiUrl":"https://graph.facebook.com",
"apiVersion":"v2.4",
"accessToken":"[TOKEN HERE]",
"userId":"409521282507981",
"numOfFeeds":"10",
"fields":"message,id,created_time"
}

Wire Logs

As requested by @RavindraRanwala :

[2015-12-15 17:29:38,650] DEBUG - wire >> "POST /services/facebook_getFeed HTTP/1.1[\r][\n]"
[2015-12-15 17:29:38,652] DEBUG - wire >> "Accept-Encoding: gzip,deflate[\r][\n]"
[2015-12-15 17:29:38,653] DEBUG - wire >> "Content-Type: application/json[\r][\n]"
[2015-12-15 17:29:38,653] DEBUG - wire >> "Content-Length: 207[\r][\n]"
[2015-12-15 17:29:38,663] DEBUG - wire >> "Host: [SERVER_IP]:8280[\r][\n]"
[2015-12-15 17:29:38,663] DEBUG - wire >> "Connection: Keep-Alive[\r][\n]"
[2015-12-15 17:29:38,664] DEBUG - wire >> "User-Agent: Apache-HttpClient/4.1.1 (java 1.5)[\r][\n]"
[2015-12-15 17:29:38,665] DEBUG - wire >> "[\r][\n]"
[2015-12-15 17:29:38,666] DEBUG - wire >> "{[\n]"
[2015-12-15 17:29:38,666] DEBUG - wire >> ""apiUrl":"https://graph.facebook.com",[\n]"
[2015-12-15 17:29:38,667] DEBUG - wire >> ""apiVersion":"v2.4",[\n]"
[2015-12-15 17:29:38,668] DEBUG - wire >> ""accessToken":"[ACCESS_TOKEN]",[\n]"
[2015-12-15 17:29:38,669] DEBUG - wire >> ""userId":"409521282507981",[\n]"
[2015-12-15 17:29:38,669] DEBUG - wire >> ""numOfFeeds":"10",[\n]"
[2015-12-15 17:29:38,679] DEBUG - wire >> ""fields":"message,id,created_time"[\n]"
[2015-12-15 17:29:38,679] DEBUG - wire >> "}"
[2015-12-15 17:29:38,812]  INFO - LogMediator To: /services/facebook_getFeed, MessageID: urn:uuid:27814841-6067-466c-b55f-b7fdd67c8d3b, Direction: request, Payload: {
"apiUrl":"https://graph.facebook.com",
"apiVersion":"v2.4",
"accessToken":"[ACCESS_TOKEN]",
"userId":"409521282507981",
"numOfFeeds":"10",
"fields":"message,id,created_time"
}
[2015-12-15 17:29:38,909]  INFO - TimeoutHandler This engine will expire all callbacks after : 120 seconds, irrespective of the timeout action, after the specified or optional timeout
[2015-12-15 17:29:40,267] DEBUG - wire << "GET /409521282507981/feed?limit=10&access_token=[ACCESS_TOKEN]&fields= HTTP/1.1[\r][\n]"
[2015-12-15 17:29:40,269] DEBUG - wire << "Accept-Encoding: gzip,deflate[\r][\n]"
[2015-12-15 17:29:40,278] DEBUG - wire << "Content-Type: application/json[\r][\n]"
[2015-12-15 17:29:40,279] DEBUG - wire << "Host: graph.facebook.com:443[\r][\n]"
[2015-12-15 17:29:40,280] DEBUG - wire << "Connection: Keep-Alive[\r][\n]"
[2015-12-15 17:29:40,281] DEBUG - wire << "User-Agent: Synapse-PT-HttpComponents-NIO[\r][\n]"
[2015-12-15 17:29:40,281] DEBUG - wire << "[\r][\n]"
[2015-12-15 17:29:40,554] DEBUG - wire >> "HTTP/1.1 200 OK[\r][\n]"
[2015-12-15 17:29:40,557] DEBUG - wire >> "Access-Control-Allow-Origin: *[\r][\n]"
[2015-12-15 17:29:40,558] DEBUG - wire >> "Content-Type: text/javascript; charset=UTF-8[\r][\n]"
[2015-12-15 17:29:40,559] DEBUG - wire >> "X-FB-Trace-ID: E5Iewv3ixeR[\r][\n]"
[2015-12-15 17:29:40,560] DEBUG - wire >> "X-FB-Rev: 2091546[\r][\n]"
[2015-12-15 17:29:40,561] DEBUG - wire >> "ETag: "ca13846784e4a3383090ca79b7a295fd23aa5641"[\r][\n]"
[2015-12-15 17:29:40,561] DEBUG - wire >> "Pragma: no-cache[\r][\n]"
[2015-12-15 17:29:40,562] DEBUG - wire >> "Cache-Control: private, no-cache, no-store, must-revalidate[\r][\n]"
[2015-12-15 17:29:40,563] DEBUG - wire >> "Facebook-API-Version: v2.5[\r][\n]"
[2015-12-15 17:29:40,563] DEBUG - wire >> "Expires: Sat, 01 Jan 2000 00:00:00 GMT[\r][\n]"
[2015-12-15 17:29:40,564] DEBUG - wire >> "Vary: Accept-Encoding[\r][\n]"
[2015-12-15 17:29:40,565] DEBUG - wire >> "Content-Encoding: gzip[\r][\n]"
[2015-12-15 17:29:40,565] DEBUG - wire >> "X-FB-Debug: G+b1bsKrSFxPDc4u1RDGIpHACsTbiJV6n1nGgNfMYwc0IqGO2fif1pnr9/o/dhQ1DLSpl7pXheknjzMwvH948w==[\r][\n]"
[2015-12-15 17:29:40,575] DEBUG - wire >> "Date: Tue, 15 Dec 2015 06:29:46 GMT[\r][\n]"
[2015-12-15 17:29:40,576] DEBUG - wire >> "Connection: keep-alive[\r][\n]"
[2015-12-15 17:29:40,578] DEBUG - wire >> "Content-Length: 814[\r][\n]"
[2015-12-15 17:29:40,579] DEBUG - wire >> "[\r][\n]"
[2015-12-15 17:29:40,580] DEBUG - wire >> "[0x1f][0x8b][0x8][0x0][0x0][0x0][0x0][0x0][0x0][0x3][0xad][0x93][0xef]o[0xa3]6[0x18][0xc7][0xff][0x15][0xc4][0x8b]J[0xd3]z[0x89]⌂[0x3][0x96][0xaa][0x89]$M[0xbb][0xb4][0xd7]6[0x97]\[0xda][0xb0]L[0x91]K[0x1c]B![0x98][0x82][0x9][0xcd]z[0xfd][0xdf]g[0xba]{Q[0xe5][0xa6]p[0x9a][0x6][0xbc][0xb0][0x85][0xad][0xcf][0xd7][0x1f]?[0xcf][0xab][0xbd][0x12]Z[0xd8][0xfc][0x8f]W{+[0xcb]RD[0xd2][0xe6][0xf6
3][0x97][0x96][0xf9][0x84]u[0xa5]D*,[0xfe][0x8b]}j[0x87][0x85][0x14]Z[0xae][0x96]:[0xde]6k[0x10][0x80][0xf8][0x13][0x84][0x9f][0x90];[0x5][0x88][0x3][0xc8][0xb1][0xfb]+0[0x8f]Y[0x19][0xaf][0xcc]⌂[0x2]<[0x8a] r[0x11][0x5][0x8e][0xe7][0xc2]%[0xc4].t[0x10]c[0x18][0x1][0x87]B[0x97][0xd8]o[0xa7][0x1f][0x91][0x13][0xb5][0x95]z[0x13]gQ[0xc3][0xad][0xb]e[0x6][0x1b]Y[0xc8][0xce]q0[0xe0][0x80]q[0xe0][0x1d][0x5][0x13][0x88] c[0x84][0xb9][0x1
][0x10]y[0x7][0xdc]QUjK[0xcb]R7[0xe8]T[0x85]B[0xc7]*[0xb3]D$[0xe2][0xac][0xd3]9Fw[0xa6][0x8]sJ8[0xa6][0xed]t[0xc]IC'[0xc][0x1e][0xd0][0xef][\n]"
[2015-12-15 17:29:40,592] DEBUG - wire >> "[0xa9][0xf5][0xde]*k)[0xb5][0x95][0xa7]"[0x94][0x96]V[0xd6]F[0xec][0xa4][0x95]VY[0xb8]i[\r][0x80][0x10][0x87][0xc7][0xbd]7[0x1](1/u[0xcc][0xdc]=[0x8][0xf0][0xbb]U[0x8b][0xd2]j[0xc4]'[0x8d][0x0][0xf1][0xa8]*[0xdd]L[0xcb][0xf].B][0x89]4[0xdd][0xb7]E[0x81][0x88]#[0xa7]5[\n]"
[2015-12-15 17:29:40,595] DEBUG - wire >> "![0xd8]3Q<[0x0][0xf0]A[0x94][0xdb][0x8d]%[0xb2][0x95][0xb5][0x15][0xfb]Gy[0x18][0xc1][0x94][0x84]L[0xd3][0x9f][0xb8][0x10]@9[0x81][0xad]!0[0x81][0xc]![0xc2][0x98]w[0x18][0xc2]7[0x9][0xc2][0x8d][0xc][0x13][0xeb][0x7][0x11][0xc7][0xd1][0x88]c[0xc2]I{-[0x98]6[0xf0]L-P[0xec][0xb2][0x1f][0xce]⌂[0xfa].@[0xa4][0xa5]2[0x3][0xa5]M[0xfd][[0xb9]2[0xe5][0xb9].[0xd4][0xd6]d[0x91]VU[0xca][0xa2]E[0x81][
0x4][0x8f][0xa3]v[0x5]f[\n]"
[2015-12-15 17:29:40,596] DEBUG - wire >> ")uMq[0x1e][0xd6][0xa4][0xff][0x1]*[0xde][0x91]mD[0xe0]p[0xc4]Z[0x89][0xc0][0x81][0xa0]!:[0x18][0x1d][0x12][0x9b]S[0xff]T[0xc7]AN[0xcd][0xd7][0xde][0xef][0x0]3dX[0x18][0xb8][0xc0]~[0xfb][0xf3][0xd4][0xce]Ed[0xca][0xdb][0xe6][0xaf]v^[0xc8]][0xac][0xaa][0xd2]l[0xdc]h[0x9d][0x97]|[0xd1]]t[0xa3]B[0xe4][0x9b][0xce][0xda][0xb4][0xdf][0xa3]RI'T[n[0xea]t[0xd1][0xdd][0xa1][0xe]]t[0xf][0x0][0x8b][0x
xca][0xd5]oi[0xbc][0x8d][0xf5][0x19][0x4]'e[0x9c][0x85][0xf2][0xcc][0xdc]+e[0xc0],uOD[0x18][0x9a][0xa3]-[0xb5]Jdv[0x6][0x1d][0xe8]0[0x8][0x8c]l[0x88][0x18]`[0xee][0xb7]Y[0xaa][0xcb][0xde]6z[0xca][0xa6][0x18][0xe6]j[0xb6][0xbf][0xf4][0x96]U[0x8][0x7][0xe]B[0xf5][0xc9]r[0xf9]O[0xd0][0xef]{e[0x16].[0xfd][0x95]?N@[0x8][0x8a]z[0x94][0x8c][0xb0]?q[0x82][0xde][0xa4][0x1e]^[0x87][0xec]9[0xae]f~[0x92]D[0x14][0xf5]\[0x15][0xf4][0xcb]"[0xbc]
]d=[0xbc]x[0x92][0xee]\&[0xdb][0xfb]r[0xf0][0xbc]&[0x81]/[0xf4]|[0x15][0x5][0xfe][0xe7]/[0x8f][0xf1][0x1d]<⌂[0xa9][0xf7]+_EW[0x15]c[0x13]W[0xcb]$y[0xbe][0x9a]{[0xeb][0x9b][0x88][0xc2]r[0x1a][0xf4][0xf2][0xcf][0xf7][0xa3]][0x1d][0xc][0x82]A[0x13][0xe5][0xbb][0xa9]3hLg[0xf2]E[0xff]⌂[0xca][0xfe][0xbb][0xa1]*[0xd3]q[0xfa].[0x9b][0x9a].[0x82][0xde][0xbf]+[0x1b][0xf4][0xc7][0xf2][0xe6]|>[0xc3]t|[[0x8f]]q3[0x8c][0xf1][0xae][0x9f][0x93][0
xe8]k[0x8c]/[0xef][0xee]f[0xe7][0xc9][0xa4][0xbe][0x8c][0x3]?[0xbd][0xdf][0xe5]_[0xfb]/[0xfb][0xb1][0x1f][0xf4][0xc9]5[0xb9][0x9e][0x5]f0[0xdd][0x8f]Y[0xe0][0xc3][0xc9][0xee]<{[0xc8].[0xd4]z>[0xa4]^[0xc8]*⌂=[0x99][0xc6][0xf][0x4][0xdc]#:q[0x9d][0xa2][0xff][0xc4]n[0x91][0xfc][0xcb][0x1b]^[0xd4][0xf]Q[0xa3][0xcc]~{[0xfb][0x1b][0x92][0x12][0xdf])M[0x7][0x0][0x0]"
[2015-12-15 17:29:40,674] ERROR - RelayUtils Error while building Passthrough stream
org.apache.axiom.om.OMException: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:296)
        at org.apache.axiom.om.impl.llom.OMDocumentImpl.getOMDocumentElement(OMDocumentImpl.java:109)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:570)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:566)
        at org.apache.synapse.transport.passthru.util.DeferredMessageBuilder.getDocument(DeferredMessageBuilder.java:149)
        at org.apache.synapse.transport.passthru.util.RelayUtils.builldMessage(RelayUtils.java:133)
        at org.apache.synapse.transport.passthru.util.RelayUtils.buildMessage(RelayUtils.java:98)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:72)
        at org.apache.synapse.mediators.template.InvokeMediator.mediate(InvokeMediator.java:154)
        at org.apache.synapse.mediators.base.SequenceMediator.mediate(SequenceMediator.java:267)
        at org.apache.synapse.core.axis2.Axis2SynapseEnvironment.mediateFromContinuationStateStack(Axis2SynapseEnvironment.java:679)
        at org.apache.synapse.core.axis2.Axis2SynapseEnvironment.injectMessage(Axis2SynapseEnvironment.java:244)
        at org.apache.synapse.core.axis2.SynapseCallbackReceiver.handleMessage(SynapseCallbackReceiver.java:529)
        at org.apache.synapse.core.axis2.SynapseCallbackReceiver.receive(SynapseCallbackReceiver.java:172)
        at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:180)
        at org.apache.synapse.transport.passthru.ClientWorker.run(ClientWorker.java:247)
        at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at com.ctc.wstx.sr.StreamScanner.throwUnexpectedChar(StreamScanner.java:639)
        at com.ctc.wstx.sr.BasicStreamReader.nextFromProlog(BasicStreamReader.java:2052)
        at com.ctc.wstx.sr.BasicStreamReader.next(BasicStreamReader.java:1134)
        at org.apache.axiom.util.stax.wrapper.XMLStreamReaderWrapper.next(XMLStreamReaderWrapper.java:225)
        at org.apache.axiom.util.stax.dialect.DisallowDoctypeDeclStreamReaderWrapper.next(DisallowDoctypeDeclStreamReaderWrapper.java:34)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.parserNext(StAXOMBuilder.java:681)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:214)
        ... 19 more
[2015-12-15 17:29:40,902] ERROR - TemplateMediator Error while building message
org.apache.axis2.AxisFault: Error while building Passthrough stream
        at org.apache.synapse.transport.passthru.util.RelayUtils.handleException(RelayUtils.java:283)
        at org.apache.synapse.transport.passthru.util.RelayUtils.builldMessage(RelayUtils.java:142)
        at org.apache.synapse.transport.passthru.util.RelayUtils.buildMessage(RelayUtils.java:98)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:72)
        at org.apache.synapse.mediators.template.InvokeMediator.mediate(InvokeMediator.java:154)
        at org.apache.synapse.mediators.base.SequenceMediator.mediate(SequenceMediator.java:267)
        at org.apache.synapse.core.axis2.Axis2SynapseEnvironment.mediateFromContinuationStateStack(Axis2SynapseEnvironment.java:679)
        at org.apache.synapse.core.axis2.Axis2SynapseEnvironment.injectMessage(Axis2SynapseEnvironment.java:244)
        at org.apache.synapse.core.axis2.SynapseCallbackReceiver.handleMessage(SynapseCallbackReceiver.java:529)
        at org.apache.synapse.core.axis2.SynapseCallbackReceiver.receive(SynapseCallbackReceiver.java:172)
        at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:180)
        at org.apache.synapse.transport.passthru.ClientWorker.run(ClientWorker.java:247)
        at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.axiom.om.OMException: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:296)
        at org.apache.axiom.om.impl.llom.OMDocumentImpl.getOMDocumentElement(OMDocumentImpl.java:109)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:570)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.getDocumentElement(StAXOMBuilder.java:566)
        at org.apache.synapse.transport.passthru.util.DeferredMessageBuilder.getDocument(DeferredMessageBuilder.java:149)
        at org.apache.synapse.transport.passthru.util.RelayUtils.builldMessage(RelayUtils.java:133)
        ... 14 more
Caused by: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '{' (code 123) in prolog; expected '<'
 at [row,col {unknown-source}]: [1,1]
        at com.ctc.wstx.sr.StreamScanner.throwUnexpectedChar(StreamScanner.java:639)
        at com.ctc.wstx.sr.BasicStreamReader.nextFromProlog(BasicStreamReader.java:2052)
        at com.ctc.wstx.sr.BasicStreamReader.next(BasicStreamReader.java:1134)
        at org.apache.axiom.util.stax.wrapper.XMLStreamReaderWrapper.next(XMLStreamReaderWrapper.java:225)
        at org.apache.axiom.util.stax.dialect.DisallowDoctypeDeclStreamReaderWrapper.next(DisallowDoctypeDeclStreamReaderWrapper.java:34)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.parserNext(StAXOMBuilder.java:681)
        at org.apache.axiom.om.impl.builder.StAXOMBuilder.next(StAXOMBuilder.java:214)
        ... 19 more
[2015-12-15 17:29:40,991] ERROR - LogMediator To: http://www.w3.org/2005/08/addressing/anonymous, WSAction: , SOAPAction: , MessageID: urn:uuid:85e6d771-e7c6-4764-8431-01e0fb4782e6, Direction: request, MESSAGE = Executing default 'fault' sequence, ERROR_CODE = 0, ERROR_MESSAGE = Error while building message, Envelope: <?xml version='1.0' encoding='utf-8'?><soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
nv:Body/></soapenv:Envelope>

Strainy · Accepted Answer

Many thanks @RavindraRanwala and @ThusithaThilinaDayaratne for leading me to the resolution of this issue.

After consulting the wire logs, it appears the Facebook API is responding with text/javascript content type. I had no message builder/formatter enabled within the ESB to handle this content type, so the process ended up failing. All I needed to do was open up my axis2.xml configuration file and append the following message builders:

<!--JSON Message Builders-->
<messageBuilder contentType="application/json" class="org.apache.synapse.commons.json.JsonStreamBuilder"/>
<messageBuilder contentType="text/javascript" class="org.apache.synapse.commons.json.JsonStreamBuilder"/>

... and formatters:

<!--JSON Message Formatters-->
<messageFormatter contentType="application/json" class="org.apache.synapse.commons.json.JsonStreamFormatter"/>
<messageFormatter contentType="text/javascript" class="org.apache.synapse.commons.json.JsonStreamFormatter"/>

The most up to date documentation on the Facebook connector seems to be at the GitHub page, where all the necessary builder/formatter information is clarified:

https://github.com/wso2/esb-connectors/tree/master/facebook/facebook-connector/facebook-connector-1.0.0

WSO2 ESB: Unexpected Character Error when submitting JSON to Proxy Service

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WSO2 ESB: Unexpected Character Error when submitting JSON to Proxy Service

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wso2-esb

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Wire Logs

Strainy

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