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conn.setRequestMethod("GET");这句抛出java.net.ProtocolException: Connection already established异常

conn.setRequestMethod("GET");这句抛出java.net.ProtocolException: Connection already established异常。如果不设置，conn.setRequestProperty("Range", "bytes=" + start + "-" + mThreadInfo.getEnd());这句会抛java.lang.IllegalStateException: Cannot set request property after connection is made这个异常，这是什么原因？