feat(cli): added build field to cdk.json

packages/aws-cdk/README.md

@@ -464,6 +464,7 @@ Some of the interesting keys that can be used in the JSON configuration files:
 ```json5
 {
     "app": "node bin/main.js",        // Command to start the CDK app      (--app='node bin/main.js')
+    "build": "mvn package",           // Specify pre-synth build           (no command line option)
     "context": {                      // Context entries                   (--context=key=value)
         "key": "value"
     },
@@ -473,6 +474,12 @@ Some of the interesting keys that can be used in the JSON configuration files:
 }
 ```
 
+If specified, the command in the `build` key will be executed immediately before synthesis.
+This can be used to build Lambda Functions, CDK Application code, or other assets. 
+`build` cannot be specified on the command line, and must be specified in either
+the Project configuration or the User configuration. The command specified
+in `build` will be executed by the "watch" process before deployment.
+
 ### Environment
 
 The following environment variables affect aws-cdk:

packages/aws-cdk/lib/api/cxapp/exec.ts

@@ -46,6 +46,11 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
   debug('context:', context);
   env[cxapi.CONTEXT_ENV] = JSON.stringify(context);
 
+  const build = config.settings.get(['build']);
+  if (build) {
+    await exec([build]);
+  }
+
   const app = config.settings.get(['app']);
   if (!app) {
     throw new Error(`--app is required either in command-line, in ${PROJECT_CONFIG} or in ${USER_DEFAULTS}`);
@@ -74,7 +79,7 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
 
   debug('env:', env);
 
-  await exec();
+  await exec(commandLine);
 
   return createAssembly(outdir);
 
@@ -91,7 +96,7 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
     }
   }
 
-  async function exec() {
+  async function exec(commandAndArgs: string[]) {
     return new Promise<void>((ok, fail) => {
       // We use a slightly lower-level interface to:
       //
@@ -103,7 +108,9 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
       //   anyway, and if the subprocess is printing to it for debugging purposes the
       //   user gets to see it sooner. Plus, capturing doesn't interact nicely with some
       //   processes like Maven.
-      const proc = childProcess.spawn(commandLine[0], commandLine.slice(1), {
+      const command = commandAndArgs[0];
+      const args = commandAndArgs.slice(1);
+      const proc = childProcess.spawn(command, args, {
         stdio: ['ignore', 'inherit', 'inherit'],
         detached: false,
         shell: true,

packages/aws-cdk/test/api/exec.test.ts

@@ -194,6 +194,27 @@ test('application set in --app is `*.js` and executable', async () => {
   await execProgram(sdkProvider, config);
 });
 
+test('cli throws when the `build` script fails', async () => {
+  config.settings.set(['build'], 'fake-command');
+  mockSpawn({
+    commandLine: ['fake-command'],
+    exitCode: 127,
+  });
+
+  await expect(execProgram(sdkProvider, config)).rejects.toEqual(new Error('Subprocess exited with error 127'));
+}, TEN_SECOND_TIMEOUT);
+
+test('cli does not throw when the `build` script succeeds', async () => {
+  config.settings.set(['build'], 'real-command');
+  mockSpawn({
+    commandLine: ['real-command'],
+    exitCode: 0,
+  });
+
+  await expect(execProgram(sdkProvider, config)).resolves;
+}, TEN_SECOND_TIMEOUT);
+
+
 function writeOutputAssembly() {
   const asm = testAssembly({
     stacks: [],