Optimize the way of generating XDR classes.

Makefile

@@ -0,0 +1,37 @@
+XDRS = xdr/Stellar-SCP.x \
+xdr/Stellar-ledger-entries.x \
+xdr/Stellar-ledger.x \
+xdr/Stellar-overlay.x \
+xdr/Stellar-transaction.x \
+xdr/Stellar-types.x \
+xdr/Stellar-contract-env-meta.x \
+xdr/Stellar-contract-meta.x \
+xdr/Stellar-contract-spec.x \
+xdr/Stellar-contract.x \
+xdr/Stellar-internal.x \
+xdr/Stellar-contract-config-setting.x
+
+XDRGEN_COMMIT=472eac8c5609d310d48bd8eed68aa24d2626df53
+XDRNEXT_COMMIT=e372df9f677961aac04c5a4cc80a3667f310b29f
+
+.PHONY: xdr xdr-clean xdr-update
+
+xdr-generate: $(XDRS)
+	docker run -it --rm -v $$PWD:/wd -w /wd ruby /bin/bash -c '\
+		gem install specific_install -v 0.3.8 && \
+		gem specific_install https://github.com/stellar/xdrgen.git -b $(XDRGEN_COMMIT) && \
+		xdrgen \
+			--language java \
+			--namespace org.stellar.sdk.xdr \
+			--output src/main/java/org/stellar/sdk/xdr/ \
+			$(XDRS)'
+	./gradlew :spotlessApply
+
+xdr/%.x:
+	curl -Lsf -o $@ https://raw.githubusercontent.com/stellar/stellar-xdr/$(XDRNEXT_COMMIT)/$(@F)
+
+xdr-clean:
+	rm xdr/*.x || true
+	rm src/main/java/org/stellar/sdk/xdr/*.java || true
+
+xdr-update: xdr-clean xdr-generate

readme.md

@@ -40,7 +40,7 @@ For information on how to contribute, please refer to our [contribution guide](h
 java-stellar-sdk is licensed under an Apache-2.0 license. See the [LICENSE](https://github.com/stellar/java-stellar-sdk/blob/master/LICENSE) file for details.
 
 ## xdr to jave code generation
-All the java source files in org.stellar.sdk.xdr package are generated using the `xdrgen` command from the [stellar/xdrgen](https://github.com/stellar/xdrgen) 
-```
-xdrgen -o ./src/main/java/org/stellar/sdk/xdr -l java -n org.stellar.sdk.xdr ./xdr/*.x
+All the java source files in `org.stellar.sdk.xdr` package are generated using the following command:
+```bash
+make xdr-update
 ```