Merge pull request #859 from MDhondt/get-revert-reason

Retrieve revert reason for EthCall

core/src/main/java/org/web3j/protocol/core/methods/response/EthCall.java

@@ -1,12 +1,42 @@
 package org.web3j.protocol.core.methods.response;
 
+import java.util.Collections;
+import java.util.List;
+
+import org.web3j.abi.FunctionReturnDecoder;
+import org.web3j.abi.TypeReference;
+import org.web3j.abi.datatypes.Type;
+import org.web3j.abi.datatypes.Utf8String;
+import org.web3j.abi.datatypes.generated.AbiTypes;
 import org.web3j.protocol.core.Response;
 
 /**
  * eth_call.
  */
 public class EthCall extends Response<String> {
+
+    // Numeric.toHexString(Hash.sha3("Error(string)".getBytes())).substring(0, 10)
+    private static final String errorMethodId = "0x08c379a0";
+
+    @SuppressWarnings("unchecked")
+    private static final List<TypeReference<Type>> revertReasonType = Collections.singletonList(
+            TypeReference.create((Class<Type>) AbiTypes.getType("string")));
+
     public String getValue() {
         return getResult();
     }
-}
+
+    public boolean reverts() {
+        return getValue() != null && getValue().startsWith(errorMethodId);
+    }
+
+    public String getRevertReason() {
+        if (reverts()) {
+            String hexRevertReason = getValue().substring(errorMethodId.length());
+            List<Type> decoded = FunctionReturnDecoder.decode(hexRevertReason, revertReasonType);
+            Utf8String decodedRevertReason = (Utf8String) decoded.get(0);
+            return decodedRevertReason.getValue();
+        }
+        return null;
+    }
+}

core/src/test/java/org/web3j/protocol/core/ResponseTest.java

@@ -70,6 +70,7 @@
 
 import static org.hamcrest.CoreMatchers.equalTo;
 import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
@@ -504,6 +505,38 @@ public void testEthCall() {
 
         EthCall ethCall = deserialiseResponse(EthCall.class);
         assertThat(ethCall.getValue(), is("0x"));
+        assertFalse(ethCall.reverts());
+        assertNull(ethCall.getRevertReason());
+    }
+
+    @Test
+    public void testEthCallReverted() {
+        buildResponse(
+                "{\n"
+                        + "  \"id\":1,\n"
+                        + "  \"jsonrpc\": \"2.0\",\n"
+                        + "  \"result\": \"0x08c379a0"
+                           + "0000000000000000000000000000000000000000000000000000000000000020"
+                           + "00000000000000000000000000000000000000000000000000000000000000ee"
+                           + "536f6c696469747920757365732073746174652d726576657274696e67206578"
+                           + "63657074696f6e7320746f2068616e646c65206572726f72732e205468652072"
+                           + "6571756972652066756e6374696f6e2073686f756c6420626520757365642074"
+                           + "6f20656e737572652076616c696420636f6e646974696f6e732c207375636820"
+                           + "617320696e707574732c206f7220636f6e747261637420737461746520766172"
+                           + "6961626c657320617265206d65742c206f7220746f2076616c69646174652072"
+                           + "657475726e2076616c7565732066726f6d2063616c6c7320746f206578746572"
+                           + "6e616c20636f6e7472616374732e000000000000000000000000000000000000\"\n"
+                        + "}"
+        );
+
+        EthCall ethCall = deserialiseResponse(EthCall.class);
+        assertTrue(ethCall.reverts());
+        assertThat(ethCall.getRevertReason(), is("Solidity uses state-reverting exceptions to "
+                                               + "handle errors. The require function should be "
+                                               + "used to ensure valid conditions, such as inputs, "
+                                               + "or contract state variables are met, or to "
+                                               + "validate return values from calls to "
+                                               + "external contracts."));
     }
 
     @Test