https://developers.google.com/protocol-buffers/protobuf-java/: Core Protocol Buffers library. Protocol Buffers are a way of encoding structured data in an efficient yet extensible format. 
public final class com.google.protobuf.RpcUtil$AlreadyCalledException extends java.lang.RuntimeException
  minor version: 0
  major version: 59
  flags: flags: (0x0031) ACC_PUBLIC, ACC_FINAL, ACC_SUPER
  this_class: com.google.protobuf.RpcUtil$AlreadyCalledException
  super_class: java.lang.RuntimeException
{
  private static final long serialVersionUID;
    descriptor: J
    flags: (0x001a) ACC_PRIVATE, ACC_STATIC, ACC_FINAL
    ConstantValue: 5469741279507848266

  public void <init>();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
        start local 0 // com.google.protobuf.RpcUtil$AlreadyCalledException this
         0: .line 123
            aload 0 /* this */
            ldc "This RpcCallback was already called and cannot be called multiple times."
            invokespecial java.lang.RuntimeException.<init>:(Ljava/lang/String;)V
         1: .line 124
            return
        end local 0 // com.google.protobuf.RpcUtil$AlreadyCalledException this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    2     0  this  Lcom/google/protobuf/RpcUtil$AlreadyCalledException;
}
SourceFile: "RpcUtil.java"
NestHost: com.google.protobuf.RpcUtil
InnerClasses:
  public final AlreadyCalledException = com.google.protobuf.RpcUtil$AlreadyCalledException of com.google.protobuf.RpcUtil